Need help diagnosing lathe threading problem

I'm trying to thread on my lathe (an internal thread, but it doesn't matter, I think). My threading dial is broken, so here is how I'm doing it:
- compound at -29 1/2 degrees, boring bar parallel to ways - made a "scratch" pass to verify tpi (8) - set cross & compound dials to 0 - advance compound (.025 IIRC), engage half-nuts & make a pass - stop motor at end of pass, leaving half-nuts engaged - advance cross slide, reverse motor & back out boring bar, past beginning of thread - return cross slide to 0, advance compound & make another pass by running motor forward
The problem: I am not picking up the thread! %$%&*^&*...!!!
Nothing is ever dis-engaged, not the half nuts, not the threading-turning selector, not the forward-reverse tumbler, nothing. The motor is run forward & reverse, the cross slide is advanced and returned, and the compound is advanced. The tool post, boring bar, & bit are all tight, as is the chuck on the spindle and the work in the chuck.
What is going on!? The only thing that I can think of is a broken pin in the gear train that is allowing a gear to slip on its shaft. Any other possibilities?
Thanks, Bob
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Bob Engelhardt wrote:

Wait a minute ... I was wrong - the bit was not tight in the bar. It was slipping back. After tightening it, it's catching the thread properly.
One dope smack for me, Bob
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