Varys
Qyburn took over or destroyed his network of "little birds". He essentially no longer has a spy network in Westeros. He admits as much before they even depart Essos.
Bran
He's never demonstrated a willingness to use his ability to do tactical reconnaissance. As he says in this episode, he lives mostly in the past. Anything he does do to help seems to be subtle: as the Three-Eyed Raven he prefers people to maintain assurances that they act of their own will and succeed by their own power. So subtle that we see him warging mid-battle as if it was relevant to something, but we're never shown any impact of it other than to let us watch the Night King command the wights to make a bridge over the pits with their bodies.
Either everyone's been respectful of his distance in these matters, or they've utterly failed to move him to change his mind. Even threats of torturing him probably won't do much. While he was making his way north he'd ignore hunger pains by warging into his dire wolf. Presumably if you tried to force him into helping by hurting him he'd just warg away and not care. Plus, the rest of the Starks still see him as family and would not take kindly to his being treated that way.
Tyrion
His every military and diplomatic stratagem since returning to Westeros with Dany has been a failure, with the enemy out-thinking him at every turn. He had a notable success in the defense of King's Landing against Stannis, but he's largely sputtered out as a one-hit wonder since then. Dany was about to strip him of his position as Hand over his failures, but Jorah convinced her to keep him. As such we have substantial evidence available to suggest that he is not good at military strategy and tactics. And that he consistently overestimates the effectiveness of the "think of your child" tactic.
Dragons
Dragons have disappeared from the land for some time. They are otherwise the world's only airborne combat unit. Proper airforce tactics have not been developed in the world because there has not been the need: either there was no air force available, or brute force and fire sufficed. It is only very recently that Qyburn invented anti-aircraft defenses.
As such Dany, nor anyone else, has ever had to devise a strategy that utilized their strengths while protecting their weaknesses. In part because they thought they didn't have weaknesses of any significance. The last thing to pose any true threat to them was a magical javelin champion.
Reconnaissance
The previous part may explain why they aren't using the Dragons for aerial reconnaissance, but why not non-dragon based reconnaissance, either? This one gets hard to explain as anything but bad writing, but I'll do so in the next paragraph. Early in the series, when everything could be taken from the books, the various armies did make use of reconnaissance and scouting. Jaimie lost to Robb because Robb made a clever ruse out of the knowledge that Jaimie would be relying on them. But for a while now, Dany's army in particular has never bothered to do advance scouting or intelligence gathering, and they have suffered at every turn for it.
If you want an in-world explanation: the people in charge are inept military commanders, and anyone who is better has insufficient will or power to get them to act more intelligently. To be fair, inept commanders are a real thing. There have been many examples of them throughout history, and they occur to this day, making the same sort of tactical errors we see Dany and her forces making in these episodes. So while frustrating to watch, you may simply be watching a true-to-life portrayal of inept military command. They've gotten as far as they have by sheer numbers and (otherwise) good luck.
Euron the Sniper
This dude can teleport himself and his entire fleet at will, apparently, so being able to stealth-snipe a moving, mid-air dragon from long range is hardly the most miraculous thing he's done. That said, how much time has been passing has been very unclear. It's always been unclear in the TV series, and even in GRRM's books he was often vague and non-specific about distances and timespans, precisely because he feared that some obsessed calculation-loving fan would find a glaring inconsistency if he did so. They may have had months to sail off (even outside the reach of Westerosi rumors if desired) to engage in some target practice, and so may in fact have had ample time and training to have mastered this weapon.
As mentioned in another answer, it is a long-standing naval tactic, even in eras with extensive air forces, to ambush another fleet by hiding on the other side of islands (or other visual obstructions, such as the numerous large rock-islands surrounding the main mass of Dragon Stone).
Though why anyone on Dany's side would think it smart to sail a small military contingent to a base that has been abandoned and unmonitored for months now boggles the mind. Even though Dragon Stone is supposed to be somewhat distant from Westeros, it's such a glaringly obvious ambush point given Dany's obvious attachment to it and previous landings there that it makes zero sense whatsoever to return to it in this situation. Any movement of theirs that relies on ships seems ill-advised given how Euron has consistently obliterated and out maneuvered them at every turn, establishing vast maritime superiority. But, once again: inept military commanders explains that.
The previous incarnation of the scorpions we saw were largely ineffectual, doing at most nominal damage to Drogon. Dany would as such be thinking that there are no human threats to her dragons. Qyburn, however, has sense retooled them and amped up their power substantially, according to the Inside the Episode.
