ASM G2 not working properly

[hqgc]

I have a G2 and it will never produce more than a quarter of the collection cup of skimmate. I have tried different heights with the bubble level in the riser tube and still no different.
It currently sits in 8 inches of water. The water height can be increased, just not decreased ( the skimmer has no more room to go higher).
I don't know why I can't get more out of it considering in was a $400 touch.
The only question I have is should the lid on the collection cup be air tight? I know that it isn't but even if you wanted it to be I don't think the ASM's will go air tight.
Any help would be great.
Thanx
H.