Most LiPos deliver a very high peak of voltage for the first bit of their discharge curve before settling into a flat plateau for the rest of the charge, so it's not unusual that the initial burst of juice would overcome whatever friction or binding you have in your system.
That said, you probably *do* have some source of friction or binding that you need to identify. Sure, you could throw a 14.8V 10000mAh monster at this, but you'd only be avoiding the true source of the problem, and you'd have the personal shame of knowing that your gun wasn't as efficient as it could be
Strip your gearbox to the bone and add parts one at a time. Test to see if your piston and sector gear bind, or if your piston and cylinder are binding, etc. Test your gears without any compression parts and watch everything as it moves.. You will eventually spot the source of friction or binding. These problems are hard to spot because you typically have to
partially assemble the inside of your gearbox while fully assembling the exterior (shell) of your gearbox (screws, and often motor + grip) to spot the problem or narrow it down to 1) before the sector or 2) after the sector (etc... you can choose many halfway points in this search).
On the flipside it's worth it because you will probably find a bad bearing or too-low sector or immobile tappet plate -- or whatever -- was the issue, after which you fix it and the gun is smooth as silk.