How big are your log backups? Check the size for each hour. If they are small, then you have something preventing the truncation of the log, like replication or mirroring being paused.

If the log file grows, it's because one of these

a) you don't have log backups

b) you have replication or mirroring that are preventing truncation

c) your load is increasing to the point where you need more peak log space.

d) someone manually grew it.

Are you sure you're checking the right database in sys.databases? You are sure that log backups are on the correct database?

There is no magic here, or no unexplained reason why log backups are not clearing the log.

One thing I noted is that you are backing up to the same file. This is easier to script, but if you have issues with the one file, you lose all log backups. Do yourself a favor and get a script that backs up each hour to a different file.