Log shipping may fail DUE to multiple issues can cause data long, Here are some possible reasons:

• Changes in shared folder or share access permissions – Copy Job is responsible for copying log backup from PRIMARY to the secondary server. If you have to change the shared folder permissions on the primary server, Copy job will not able to access the share of primary server to copy Log backups.
• A human error like someone deletes the T-log backup file or truncate T-log on the primary server – This is the most COMMON issue. If someone has deleted Log backup by mistake log chain will break and restore might fail. ALSO if Someone truncates the T-Log file on primary than log chain will break and new log backup will not generate.
• Low Drive free space is low on secondary – Standard approach is to have similar drive structure and space on secondary like primary. If secondary has less drive space that it may get full and impact copy or restore process.
• Low I/O, Memory, NETWORK resources  - Copy & Restore job needs resource and if you have a long list of databases then you need high resources. Secondary server with low network \ IO or memory can cause server slowness \ crash or delay in the restore.
• TUF file is missing – TUF is transaction undo file which contains active transaction details. If TUF file is deleted and you do not have a backup then you have to reset shipping.
• MSDB database is full – MSDB keeps track of log shipping & restore history. If MSDB got full then Copy & Restore jobs will start failed.