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one thing which might cause this symptom is damaged threads
simple enough to disassemble everything and check them
if damaged, chasing might be adequate to sort things
if damaged and chasing not adequate you could tap the shell out to italian
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one odd thing which can cause an adjustable cup to loosen is if bottom bracket threads to do not go far enough in toward the centre of the shell
had one case of this in me workshop where the adjustable cup was butting up against the end of the threads right at the point where the bearing adjustment was correct. this caused end loading on the cup which caused it to loosen - an odd problem indeed.
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