The SQL service packs seem to always fail more often than most when installing patches but if you do get a failure try checking all SQL related services and making sure that none of them are disabled; if they are then set them to manual.

MSSQL$....

SQLAgent$.....

SQL Server…..

The most common catch we've found is the  "MSSQL$SBSMONITORING" service that is often disabled (as it's unused)  and after being set to manual the service pack which failed previously installs successfully!