I don't know whether its a good idea to drive without them locked. One person wrote that he knew of a driver who had been badly injured when the seat back flew forward in a wreck.

If you take the seats out (not a terribly hard task), you can get at the fittings and vacuum valves. It could just be one has come loose. They aren't real complicated, just impossible to get at with seats in car.

Do the trouble shooting to see if its both seats or just one that needs your attention.

Nic