Does a 'dump tran' clear the log?
If not, then I'm guessing you have a (replication) secondary truncation point in place that isn't being moved. (should show up as a record in master..syslogshold with name = '$replication_truncation_point')
If you do have a secondary truncation point in place, and it's not moving, then you'll need to figure out why it's not moving and address the issue, eg:
- [PDB] repagent is not running
- [PDB] repagent is not able to connect to the repserver (eg, invalid login/password, repserver is down)
- [PDB] repagent is unable to send txns to the repserver because the repserver's stable queue is full
- [RDB] no repagent is configured but trunc pt exists (eg, brought over in a dump-n-load) => drop the trunc pt via 'dbcc settrunc(ltm,ignore)'