If you’re still having that problem after checking what was recommended above then you may want to make sure mainspring barrel cap is snapped and seated all the way down into it’s recess. If not it may be dragging on the plate or bridge. Also check for any screws or screw heads that might be in the path of a gear etc. People often put the wrong screws in the wrong places when assembling. In addition screws do loosen up and fall out. You’ll find them floating around between those plates much too often. Have run into this many, many times myself.
I think SteadyPin is referring to the back spin of the escape wheel that should occur. After the mainspring has completely unwound a spinning gear trains inertia should cause the train to continue spinning a bit, ‘winding’ the mainpring slightly in the opposite direction. If the watch is in good condition this tiny bit of energy will cause the escape wheel to spin backwards a few turns. This is a good indication that the movement is in good shape. As Steadypin say’s, it’s very important.
Good luck with the hunt Watchdogg,