Downhill effect
When a film contains a number of good scenes which are almost immediately followed by scenes of a far lesser quality, causing the film to slowly move downhill until there's no hope in sight. The downhill effect was made popular by Raymond Walter back in 1988 with his film "Blue Code."
"The Perks of Being a Wallflower" really suffered from the downhill effect unlike the much superior film "Me and You and Everyone We Know."