Moved to EPO forum you will have better exposure there to like users
You should upgrade agents using just a product deployment task - did you check any logs on the machines that won't upgrade?
There is no neccessery to use force switches - or even this is no possible from the ePO.
But if you want you can build your own packeges with that switches using EEDK or...
... you can simply use "Deploy Agent" with force checked:
Thanks Michal. Let me try it.