Check that the tran log backups are running. How often do they run?
Run this query for the DB with the problem. Please post what it returns.
select name, recovery_model_desc, log_reuse_wait_desc from sys.databases where name = ...
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability