Check each steps history, either

• logs are not being copied, hence no restore
• logs are being copied, but a log backup is missing, so no restores will take place

You have an LSN gap probably, this can be bridged by locating and restoring all log backups or, if no full backup has occurred against the Primary, apply a differential backup.

To check if the Primaries differential base LSN is in sync with the Secondary run the following against the Priamry and Secondary instances and return the results

select name, differential_base_lsn from sys.master_files

where database_id = db_id('logshipped_db_name') and type_desc = 'rows'