If it will run on its front and not on its back it could be a loose cap jewel resticting the balence movement, but the way you have described it it sownds like some thing stopping the power from the main spring geting through the movement.
Have you tried releasing the main spring pressure?, Open the crown guard, move the ratchet click (the bit that locks the main spring gear) so that the gear is free to rotate and while alowing the crown to rotate in your fingers alow the spring pressure to release. If fully wound the crown will spin 60 odd times untill fully unwound, Have a good look inside especialy in the area of the palet wheel and balence for any loose items like palet jewels, if it is all clear give it a couple of turns on the crown and see what happens