Sometimes soaking the seal expands it too much and doesn't allow it to gain speed. I'm assuming you've had it apart? Does the piston slide (in the receiver tube) with a gentle pressure?
If so, and all else is good, it is possible you have a bulge in the tube near the transfer port (could have been manufactured that way). This will result in a velocity decrease. Again, you need it disassembled and see what happens when you slide the piston down.
The bottom line is you need to strip it down to diagnose it.