Check the wiring from the switch to the motor. Sometimes 'stuff' under the seat can mess with things.
Clean the contacts. Look for pulled or broken wires.
If all is good, check the switch itself. Could be, may be, that something got spilled on the switch? Soda. Coffee. It happens. That'll gunk up the contacts.
Those switches do break. The 6-way more often than not can be problematic, especially that center 4-way toggle. The return springs inside can break or they can just get tired. The switches can also break at the pivot inside. The switch contacts are bad for getting gummed up with stuff, too.
The pins on the back of the switch can break.
Gunk on the pins will prevent a complete circuit. Those pins can break off inside the female connector. They'll still be there, just no worky. Maybe occasionally, but not consistently.
The wires can break off from their connections inside that female receiver.
Many times, those switches aren't worth dickin' around with.
I don't believe that they're available from GM anymore.
Best repair for them?
Find an Escalade at the wreckers or any other top end equipped model of a GMT400 with a power'd passenger seat & grab that switch. Passenger = much less usage.
GM also used the same switch in many other vehicles of the era, too.
But only after going through everything else first.