Reasons for Log Shipping Failure:
1) Jobs disabled can be a cause for LS failure.
2) Backup Share permission issues.
3) Space issues in the backup share/local copy location.
4) SQL Server Agents stopped at Primary/Standby/Monitor.
5) Manual log backup may cause LS break.
6) Recovery Model change from Full/Bulk Logged to Simple.
7) Backup/Copy/Restore Job owner change can cause permission issues and break LS.
8) Network Issues with Backup Share.
9) WITH RECOVERY statement fired at Standby server can bring secondary database ONLINE breaking LS.
10) Service Account changes can lead to permission issues.
11) Log backups getting corrupted/deleted.
12) Backup schedule if changed can cause lot of delay which might raise an alert.
13) Timezone differences between servers(Primary/Standby/Monitor) can cause issues in the backup/restore/copy timings and can cause an issue.
14) Adding a file on Primary (with no similar path on standby) will break log shipping.
15) If TUF file gets corrupted breaks log shipping.