Bad air seal with the hop up rubber. Try a DIY mod I did on mine: Cover the part of the barrel where the hop rubber end with PTFE and use nail polish to seal it(not 100% required),then put the hop rubber on. Then wrap the end of the rubber with dental floss.
If that does not work a new hop rubber for a better seal is in order.
I think it's more likely that the hop is just blocking the barrel too much so it takes more of the air to just get it past the rubber.
I've only got .3s and .43s though, I could probably get away with it as it is if I just get a 500fps spring and use the .43s... We'll have to wait and see, 'cos there's an M150 in the post as we speak.
If that does not work a new hop rubber for a better seal is in order.
Did you try .36g Bbs?
Will dig for some ASSF threads for more help.