One of the things that commonly occurs is that the stack shut-off
tube or slide has a leak, and the stack shut-off is trying to close
while the unit is in "play".

Remove the tube from this unit, located on the bottom left hand side
of the player.  Close off the nipple with your thumb and see if the
stack does not perform any better.  Another method is to use the tube
and suck on it by mouth from the pouch shut-off while the player is
running.

If these things don't work than perhaps your stack needs rebuilding.

Jim Jelinek