It certainly sounds like your tile shower is a problem.
Grout is not waterproof and seats are a real problem area.
IMHO the Kerdi system is the best water proofing available today and is what I would use in my own home...
You want to find a tile contractor that uses this method...
Check this link out...
Schluter Kerdi Shower, Accessible, Mold-free, Leakproof