My first guess would be dirty/oxidized contacts at the T-R relay, so although the amp is "keying," the RF drive applied is not getting to the cathodes.
Try cleaning the contacts using a piece of coarse paper (I usually use a dollar bill -- unless I'm feeling rich, then I might use a twenty!). A piece of paper soaked in WD-40 actually works even a bit better.
Do not EVER use any kind of file or "spark plug cleaner" type tool. A very fine (soft, almost like skin) burnishing tool is okay, but actually a dollar bill works great and is coarse enough to clean contacts well.
A government which robs Peter to pay Paul can always depend on the support of Paul.
-- George Bernard Shaw