Exactly what do you mean when you say it "won't run"?
Does it run for a while and the pendulum slowly stops?
Does it stop fairly sudden?
Tell us what you observe.
A couple of things that I would suggest is to let down the power completely, then remove the hands and the anchor. Wind the mainspring about a quarter of a turn or so. The escape wheel should begin to spin. If it doesn't then touch each wheel from the escape to the 1st wheel. If it starts to run when you touch one then there is a problem in that wheel or the adjacent one. I would look for bent arbors, pivots, teeth etc. If it does run then put the anchor back in the train. Wind it about one turn. While slowly moving the anchor pin, observe the escape action. Are the drops correct? on each and every tooth of both the the entrance and exit pallets? Make sure that you observe evey tooth of the wheel. When you do this it is very easy to see a "problem" tooth.
If everything appear to be fine then mount the movement in a stand, and hang the suspension and the pendulum. Level it in both planes. Put the movement in beat. Does it run? If not then remove the intermediate wheel. Does it run? If it does look for look for a bent hand shaft. If it still does not run then something was missed. Is the spring free in the barrel, is it coned causing it to drag and lose power.
If it runs with the movement complete in the stand for a day then mount the hands. If it does not run then there is a drag that the hands are causing.
If it still runs then mount the movement in the clock. Be careful not to overtighten the mounting screws, this will flex the plates and bind the wheels.
Hang the pendulum and check the beat.
Start the clock and it should run, if not you have missed something.
I find after going through the above steps that I'll usually find the problem.
I hope this helps. If not I am sure John will be along soon to help!