you can compare the sensitivity of your TLM193 with another microphone; record the same signal with the mics side by side; determine the level difference with identical gain settings; compare the level difference with the theoretical sensitivity difference given in the TLM193's and the other microphones manual. Then judge if there is really a sensitivity loss in the TLM193. Otherwise you would have to send the microphone in for a service check, so we can make a "real" acoustic measurement.
[I can't think of a reason why a TLM193 should loose sensitivity]
Best regards, Martin Schneider / Neumann Mic. Development