Or that they are making mistakes? There is a difference. We can all agree that dropped passes are a big problem, but again that is not something that coaches have a lot of influence on. Unforced dropped passes are a mental error by the WR. Its a matter of focus and trust. WR's not ALWAYS getting separation can be due to many different variables like defensive scheme, DB ability, and of course WR ability. There have been several times where a WR has broken his route deep and Taysom either didn't see it or didn't trust his arm to make the throw.
Football isn't rocket Science, learning the offense is not the issue. I would say that most of the players just aren't comfortable or confident with the offense yet, which is understandable.
I'm sure Tanner will be just fine running it next year.