AJ,

I've just installed a single lever Moen shower control...the problem
is that the control actually "sucks' back into the assembly resulting
in no or little water flow when the control is set to midway between
hot and cold.

I just finished installing some Moen shower faucets myself. One of them was
like yours, just a trickle of water. I fixed the problem by disassembling
the valve, and cleaning out the little pressure balance piston inside.
Probably just clogged with plumbing debris. Once it was cleaned and
reassembled, it worked fine.

Since this is described in the installation instructions, I assume it's a
fairly common problem.

Oh, one other possible cause, the handle could be on upside down.

Anthony