What sort of HPA tank? At what output pressure? Has this gun worked using that HPA tank in the past? Did you try it with a CO2 tank at all? That could certainly cause this sort of problem. CO2 comes out of the tank at a higher pressure than HPA typically does, if your HPA tank doesn't have a high enough output pressure to effectively cycle the marker, that will cause this sort of problem.
Here's a handy schematic...
Use that to make sure you've got the bottom tube in order.
My thoughts after looking at that diagram...
Meticulously clean and lube the bottom tube with 3 in 1 oil, or something similar. Not grease or gun oil. Replace the valve and striker o-rings with whatever the proper size ring is.
Check that the springs are seated properly and in good working order, perhaps give them a little stretch. You might have too much tension from the valve spring and not enough from the drive spring. That would cause recocking issues.
Make sure that your velocity adjuster is set in a reasonable range. Too far in or out can cause problems.
Lastly, move to the upper tube, and check that the ball detent is properly installed.
Somewhere in there you should get this problem licked.