Fabrizio,
Please take a look at the following post, which could be tracking a very similar issue with yours, and see if any of the suggestions made there would be relevant to your problem:
http://devcentral.f5.com/Community/GroupDetails/tabid/1082223/asg/54/aft/1177189/showtab/groupforums/Default.aspx1225360
Basically, what I would suggest you to do is to run the two tasks for showing failed rules and running rules on the Health Service:
1. Failed Rules: You can run this task from SCOM Management Console :: Monitoring :: Operations Manager :: Management Server :: Management Server Health State :: select your management server :: Health Service Tasks (in the Action pane) :: Show Failed Rules and Monitors for this Health Service.
2. Running Rules: You can run this task from SCOM Management Console :: Monitoring :: Operations Manager :: Management Server :: Management Server Health State :: select your management server :: Health Service Tasks (in the Action pane) :: Show Running Rules and Monitors for this Health Service.
For 1: Make sure the F5 related workflows are NOT shown as being reported as failed rules. If they do, we'll have to investigate why did they fail to load.
For 2: Make sure you see the F5 Monitors and rules loaded and running. If not, than we may have a problem with the actual SCOM F5 Management Pack not being correctly imported in SCOM.
Julian