Shower leaks when someone is in it

We recently moved into a new house. Both of our showers leak, but only when a person is in them. They both take about 3 minutes to start leaking. The ceiling is opened below both leaks. We have redone the caulking and tested the outside of the shower by pouring water on to the floor and nothing seems to be leaking when that occurs, so it seems to be coming from inside the shower. The handles have all been checked and seem fine. Our guy can't figure it out and says the pipes seem fine...any help would be appreciated!

One is a stand up shower and one is a full tub. The full tub leak continues to leak for about 30 seconds even after the shower is turned off.

What type of material is the tub and shower made of? Sounds as if the drain connection is leaking due to deflection by the additional weight when someone is in the shower. If the units are made of fiberglass you may have cracks in the base caused by over tightening the drain connections.

Thanks for the info Rob. The stand up has a pan that seems to be plastic and the tub is porcelain.

Since the area below the units are open can you see where the drips start? It might help others here to make suggestions by seeing some pictures. Especially if you could get one as the drips start. I'm thinking two different issues because of the different materials. Are the drain lines PVC or cast iron?

Besides the added weight of someone in the shower, the water is now bouncing off their body and hitting surfaces that were not getting wet, when the shower was running un-occupied.

Still could be lousy caulking.

100% of the old caulk should be removed, all surfaces cleaned with bathroom cleaner then rubbing alcohol.
Put a fan in there for a day, to dry out the joints.
Then recaulk with silicone bath caulk.

Fiberglass surround might have rubber joints that are failing.

Valve trim plates might need caulking around, or other wall penetrations.
Tub spout, towel bars, soap dish.