I had a super small pinhole in my tube where it did not show air bubble even drowning it it a bucket of water. What did the trick was to inflate the tube and immerse in water while sqeezing about 8 inch of tune with both hand with a twisting, stretching and compression motion. Eventually it will show up.
If nothing shows up with that method, then try inflating the tube in the tire and rim to the highest psi rating and soap the air valve for at least half an hour. The leak sometimes only shows up under normal running pressure b