ASDM Real-Time Log Viewer not showing log messages

Make sure following commands are enabled

logging enable logging asdm informational

If you are seeing some messages, and some are not showing

Do a check on
# sh run | i log
to see any syslogs are kept disabled
Eg:
ASA(config)#no logging message 304001
ASA(config)#no logging message 304002
ASA(config)#no logging message 304003
ASA(config)#no logging message 304004
ASA(config)#no logging message 304005

Note: check cpu usage after enabling logging . if you are logging to multiple Syslogs , it can cause high CPU.
Also do a check why it is kept disabled before enabling the same.