This may sound sarcastic but it's not meant to be.
Do you think maybe the frame is all the way back but the new padding is making you feel like you're closer to the dash and higher up?
Never mind - just saw that towjoe already covered this...
If you still feel that the bench is not sliding fully back, you can check that it is hitting the front and back stops by measuring how much it moves.
Seems like much more, but actually the full range of adjustment on those bench seats is only 4".
It can be easily measured at the bottom of the seat. Use the position of the seat belt retractor casing in relation to the hump above the sill plate.