When taking a bath; the bath stopper keeps getting pushed back up, draining the water, --- the stopper is a 1 1/2" rubber stopper (not the mechanical kind)

Is this considered back pressure? If so, how can I correct it?

Thanks in advance

This has just got to be a older tub without a tub waste and overflow. It sounds like there's a partial blockage some where and when someone's drains a fixture it sets up a back pressure that kicks the stopper out. To prevent this you can locate the blockage and snake out the line or you can install a touch toe stopper, (see image). Good luck, tom