The seat adjusters are new and are not broken in.

Operate the seat to the full forward and the full rearward position several times in order to work the tightness out of the channels.

The adjusters are not properly lubricated.

Lubricate the channels with Lubriplate, or the equivalent.

The adjusters are binding because the channels are bent or damaged.

Replace the seat riser.

The adjusters are not in parallel alignment with each other.

1. Loosen the floor-pan attaching bolts or nuts.
2. Align the adjusters parallel on the floor pan.
3. Tighten the bolts or nuts.