Check G.Scott's recommendation 1st. That's the obvious one, most likely.
If that doesn't apply, consider flushing ASM's DB on the Standby unit. Don't forget to take a UCS backup for safety reasons: https://support.f5.com/csp/article/K6992
- When DB has been reset on Standby unit, manually enable ASM sync (the same as in 1st recommendation), and initiate a sync from Active to Standby. This initial sync could take up to 5 minutes if you have a hundred policies. After initial sync, all is back to normal.
This will eliminate blocking conflicts in ASM's MySQL DB and a seamless sync can take place. Those issues are rare, caused by very circumstantial bugs. Last time I did this procedure with BigIP v12.1 half a year ago. When you're not in a rush to restore sync, consider contacting F5 support so they can have a look and see if there are any un-addressed bugs.