Large transactional log won't shrink after transactional backup
What is the frequency of the log backups ?

Use dbcc sqlperf(logspace) to check how much % is free space in the logs.

There is something that is holding the logs up. Check the log_reuse_wait_desc for that database and take action accordingly. 

Follow the below article :

https://www.brentozar.com/archive/2016/03/my-favorite-system-column-log_reuse_wait_desc/