I've had three 1994 vans in a row, and all three have had this problem.

(1994 Dodge Grand Caravan)

The inside lock actuator on the side door feels rubbery, and sometimes when you lock the door it won't properly unlock - the door just refuses to open.

If you pound on it and work the latches in and out they will eventually open.

What is the problem here, and what is the best way to fix it?

 

Have you pulled out the latch and looked at it? Usually the latch gets all rusty and corroded from all the years of moisture and they just don't work good anymore. I remove mine, clean them with a wire brush and then soak them in oil. They seem to come around.