1991 Plymouth Laser 1991 Plymouth Laser, door key question

2 possibilities

The locks have been changed,
or
they are dirty, or were never used and need to be freed up.

2-A.
The pins are not falling into place.

You kinda need to know how a lock works for this part.
Look at a key and you will see there are different depth cuts in it.
Those different cuts correspond with different length pins in the lock cylinder.
See how the red pins are of different length?
But with the key inserted, they are all even with the blue pins.

When the red pins are held in the correct position by the correct key, the spring loaded blue pins are flush with the outer housing of the lock, and the red pins are flush with the inner cylinder of the lock
And thus the inner cylinder can be rotated.

If debris such as pocket lint, ear wax, or just plain old dirt gets into the pins and causes enough friction for the spring loaded blue pins to not be able to push the red pins back in place.
Then the red pins will be partially stuck in the hole for the blue pins and the cylinder can not turn.

Putting the key in the lock and then tapping on it with a small hammer may help until you have the chance to have the lock cleaned.

Do a search for the word "bump key" if you want to know more about how a lock cylinder works.

you might try getting some spray lube with one of those starws on it so you can spray deep into the lock.