If you can see water in the trap , then the trap is holding water and you know it's working. Even if the trap is undersized (1 1/2"; instead of 2";) it would work. So the problem is probably a leaking drain pipe, the shower drain itself (the part that is connected to the shower stall) or it's leaking where the two connect. Can't fix the pipe or the drain itself without pulling the shower out. But, if you can see a rubber or lead ring around the pipe as it sticks up into the shower drain - that can be removed and a new one put in. This is a pretty common practice in concrete shower installs.