A problem that I have run into, as far as adults learning to read music, is simply that they get ahead of themselves--they bypass the simple things because they are "too easy" and then get snagged by trying to read things that they aren't ready for yet--

Another problem with folk/pop/rock musicians, particularly with guitar players--is that they have often learned to play by copying what they hear, so they don't think about what they are doing in terms of quarter notes or eight notes, and often not even in terms of A-B-C-D-E-F-G.

Biggest problem is that adults often don't have patience to sit down each and every day to practice their reading, which is all it really takes to learn to read music.