You can quickly verify the following:
1. emoms.trc file from OMS ...gc_inst/em/EMGC_OMS1/sysman/log location.
2. Connect to repository DB as sysman and verify the following query:
select * from MGMT_BLOCKED_AGENTS where target_guid=(Select target_guid from mgmt_targets where target_name like '%<AGENT HOST NAME>%‘);
This would help you to understand what is the exact error.
One more thing to verify, from the query provided you will get the exact reason why the plugin failure has occured based on which we can understand our next actions.
There are many scenarios for the failure.