There’s no easy answer to the “how can you tell what went wrong” question, certainly not one that can be posted in a necessarily short-n-ephemeral message board. The bottom line, though, is that it takes time and experience. After seeing several thousand movies, reading much background material (biographical, technical, business) on the craft and industry in general as well as on specific productions, and working on films myself, I think I have a pretty good handle on how things are put together and I can identify the weak elements of a film, at least in a broad sense, from one or two viewings.
However, this is not always the case.
For example, I saw a thoroughly awful film very recently. I happened to see it under very secure circumstances, and I can reveal absolutely no details about it, per the ironclad agreement I signed when I received my pass. I’d very much like to identify it, in order to warn interested viewers away from it, but I can’t, so don’t ask; I’m not even going to hint.
I bring it up, though, because afterward my moviegoing companion and I were trying to figure out where the movie went wrong. It had some evident potential, but by the end the good ideas were twisted up with the bad in a catastrophic flaming trainwreck. And the question we were debating was simple: Whose fault is it?
We did this by carefully reviewing the credits, attempting to identify source material and tracking backward from there to puzzle out what elements came from where, which stuff was on the page and which stuff was contributed by the director or whomever. We discussed the filmmakers’ histories, the director and the producers and so on, considering how they usually go about making their movies: which writers do they use, how long do they stay in preproduction, what kind of input do their actors typically have, and that sort of thing.
In the end, I came to the tentative conclusion that the film, metaphorically speaking, was put in the oven before the batter was fully mixed. It is sometimes the case that a film must be put into production for financial or logistical reasons before the problems with the script have been worked out; look at The Devil’s Own, that IRA/cop movie with Harrison Ford and Brad Pitt a few years back, which was forced in front of the cameras without a fully polished script partly because the two big movie stars had established their availability just so and if the schedule slipped one or both of them would have to drop out and the project as a whole would then be endangered. Without going into specifics on the nature of the mystery movie I’m discussing, I strongly suspect the script was being worked on and known problems were being ironed out when for one reason or another they had to rush into production before they were ready. That is, I will admit, a guess. It’s an informed guess, but without additional information it’s still a guess. It’s quite possible I’m off base, and that the director fell down in the bathtub the day before shooting started and hit his/her head on the faucet and suffered brain damage and as a result created the incoherent mess I saw. Or maybe he/she is just plain incompetent, and his/her prior successes were flukes of luck. Or something else. Honestly, I don’t know. I think I know, and I’ll be looking for more information, but I’ll be surprised if I’m not at least in the ballpark.
Does that help? I know an answer like “take your time and learn, young padawan” isn’t particularly satisfying, but it’s the only honestly accurate response, in my view.