Backing up the log won't decrease the file size of the log itself. It frees space within the log of committed transactions. If your log is continuing to grow, I suspect you may have either extremely large transactions, or you have uncommitted transactions that are holding on to their space. You should check sys.dm_tran_active_transactions to see if you have open transactions. You can combine that with other DMVs such as sys.dm_exec_requests and sys.dm_exec_sql_text to see the query, blocked/blocking processes, resources waits, etc.