Rob spoke to steve pope recently .
steve put quite some emphasis on getting the seal fit just right .
it's very difficult to gauge , but if you are using the complete kit as supplied i'm guessing its just that the seal might be a bit tight .
we devised a kind of method of comparison of seal fitting by fitting the piston in the tube , standing it vertical and measuring how many tins of pellets it took to make the piston travel down the comptube .
i even made a gadget to enable us to do it no handed
i'll see if i can dig up a pic
just scroll thru this you'll get the gist..........might help