How high is the hop up turned up?
what kind of bb's are you using?
is the spring that sits in front of the hop up still there?
it could be a few things, im not sure how good that stock hop units are on ics's
here is a hop up video
How to dismount an Hop Up M4/M16 - YouTube
you dont have to fully dissasemble you can just pop off the c clip and slide the barrel and rubber out for less bits to mess with just hold the unit with the top down so the nub doesnt fall out