It does sound like the spring to me unfortunately.
You disassemble the spring mechanism to try and re-tension it, and it can also come lose in the centre so there's only enough tension for a few shots. They produce a replacement spring mechanism, I bought a few of them but they're so hard to buy, however they always ended up failing again.
If you want to try and use a pistol as a primary weapon HPA and an M4 / MP5 magazine adapter is the only reliable solution.