I can shift down into 1st, either moving or at a standstill. When I then pop it over to 2nd, it stays in 1st. I can also pop it over to 3rd, and it still stays in 1st, regardless of RPM. The only way it'll shift is by putting it into drive.

If I leave it in drive, it goes through the gears (starting with 2nd) just like it's supposed to, smooth as silk. What could be keeping it from manually shifting through 1st, 2nd, 3rd and 4th when I feel like rapping it up a little?