If your negativs are universally thin, you could have a problem with both underexposure and underdevelopment.
Take a roll of film and shoot quadruples of frames (important: in normal contrast) bracketed at EI 100, EI 200, EI 400, EI 800. Process the film according to the manufacturer's recommendations.
Find the frames with enough shadow detail for good separation. Then shoot a roll at that EI (exposure index, or 'film speed'). When you process the film cut it in thirds. Now process each third at -20%, at recommended time, and +20% of the manufacturer's recommendations to see if you can somewhat dial in a ball park of where your exposure and development needs to be for a normal situation.
By using two different films, you can't relate the results of the first film (FP4+) to what happened with the TMax.
My recipe for Tmax (replenished, which has activity about similar to in between stock and 1+1 dilution) is 8 minutes with agitation first 30s continuously, then 10s every two minutes. That's for normal contrast and at box speed (400). The 6.5 minute time by Kodak sounds accurate if you agitate every 30s or every minute.
A combination of slight underexposure and underdevelopment could result in thin negatives.
- Thomas