JCB8080 2007 Lacking power

Hi all, I have interesting problem on JCB8080. It lacks hydraulic power. It can not raise itself with boom. Arm and bucked also don't have enough force. I didn't test rest of the system. I measured pressures, main relief 300 bar, pilot 36 bar, so they are Ok. I measure pressure at the boom cylinder, pressure side 300 bar, return side 0 bar, and it still lacks power. cylinder was recently refurbished. Now story is that pump was also repaired recently and before that excavator had barely any movement at all. I spoke to one of the local JCB guys, and he tells me that flow of the pump has to be increased to get more power. Now my questions are: is it possible that after repair pump wasn't adjusted properly? How can increasing flow increase power? unless oil is bypassing somewhere and increasing flow forces more oil into bypassing and therefore putting more force on the piston? To me, if the system is correct, flow adjustment shouldn't affect power of the cylinder if there is 300 bar on it. What are your taughts? Thanks!!!