To check for slipping, (engine off) fasten your FD down on the tire with the amount of pressure you normally do, grab the middle of your handlebars (stem) or lay your chest on your seat, grab the shaft of the FD (gloved hand), turn the shaft to make the bike go forward while fighting the motion with your weight.If it spins on the tire you need more pressure, only add to where you can't spin the shaft by hand.
If that's not the problem, I've noticed with mine that bent steel channels aren't perfectly straight, with the shaft lifted off the tire, you should be able to spin the shaft and it turns a few revolutions, not come to an almost instant stop.