If you have water getting out of the shower, it is coming from these possible sources:
- failed shower pan
- improper tile substrate and backing
- plumbing (pipe or fixture) leak
You need to rule out each one. Loose tiles tell me that there is a substrate issue. Did you build the shower, or have it built by someone? If yes, what tile adhesive was used? If it was pre-mixed (also known as "mastic"), then that is the problem. Also, if the backer behind the tile was not cementitous (cement board, or equivalent), then even if the proper adhesive (thinset) was used, the drywall can get wet and mushy.
Even if the grout and caulk are perfect, if mastic or drywall was used in the shower you will eventually get leaking.
A failed shower pan takes a little more sleuthing. The wall tiles that are loose - how high up are they? If they are near the bottom, it could be you have the weep holes plugged in the shower drain, or there is no pre-slope under the tile. This would cause water to acumulate, and if the proper adhesive was not used (see above), you will have problems.
From your description I don't think you have a leaking pipe or shower fixture.
If you can post some photos of the loose tiles, and of your shower in general, people may get a better idea on what to suggest as a solution.