Bass and the bench have been struggling because of two things. This bigest one is poor matchups, the second is lack of plauoff experience.
The poor match ups are the big one. Atlanta is a strange team where they are both big and small at the same time. Horford is slightly smaller then the average C, Smith slightly smaller then the average PF, Marvin williams and T-Mac are above average SFs, Joe johnson is a large SG, and Hinrich and teague are both average PGs. Smiths ability to close on bass quickly eliminates his open Js while his foot speed is to much for him to drive around. Steimsma isn't a good enough perimeter defender to play against most of Atlanta lineups. There length at 2/3 also allows them to close out on pietrus when he is sitting on the 3pt line.