Mine wouldn't move either, finally took it out the other day. In my case I think it's mostly due to friction and lack of lubrication between the seat frame and the rail it slides in. Check it out, could be your problem. I hit it good with pb blaster and worked it back and forth with a plastic mallet a few times- still tight but better than it was. I'll grease it up good and put it back in.. Also, have someone sit on the passenger side and work it forward as your work the drivers side forward-sometimes when they're tight and your just trying to rock it forward from the drivers position it can put a bind on the mechanism and it won't move forward. One more thing, make sure that when your depressing the lever on the drivers side the connecting latch underneath the seat on the passenger side is also disengaging from the slot it locks into.

53 Chevy 210 convertible; 55 GMC pickup; 57 GMC Suburban Carryall