VSR having a nightmare

[Justjay1986]

So I'm currently working on a mates vsr compatible specna, which will not exceed 400fps no matter what I try.

Compression seems fine and the FPS is actually pretty consistent. But not going over. 

It's stock barrel and rubber (which I'll likely be changing soon, and possibly the culprit of this) but it's got a Maple leaf spring (m145) and spring guide, vsr cylinder and piston and Airsoft pro trigger set. All firing really well not a blip, apart from this sodding FPS. Am I missing something really simple here? 

Tia

Edit

The reason the hop isn't done yet, is because the rubber we gave me doesn't sit right.....don't ask.

 

[LMKipper]

Could it be that the hop isn't holding the BB long enough to generate the pressure? So the BB is on its way before the piston has generated maximum power.

 

[Justjay1986]

Possibly so, I've had sporadic problems with bb's rolling out when cocking back at times also. Frustrating me endless amounts now

 

[Justjay1986]

Update - new cylinder head, barrel and hop rubber sorted the issue lol