Hi,
Can you check alert log permissions .
It should be readable by "B" user.
if your DB owner is "A" OS user and Agent owner is "B" OS user.
We had same issue when our agent owner was not able to read alert log contents.
You can also check following
On EM On your DB Home-page in Metric collection error link below does it showing any errors.

OR you can check
DB Home page --> All Metric --> metric (Alert Log Error Status)
See if they are showin "Set" . If its set and and not able to collect than it will say . Collection error on "date"

Hope this helps....

Rgds
Parag