Can't take a compliment
You praise how they handled a hard client. They wave it off ('anyone would have'), change the subject, or point to the one thing that went wrong.
Rule out first
Cultural or family modesty norms. Praise that sounded generic or strategic. A real flaw they know about and you don't.
What could explain it
Self-verification: Praise contradicts a settled self-view, so it feels wrong rather than pleasant. They're more at ease with accurate criticism than with praise.
Discounting the positive: A habit of mind, not a wound: they discount good news about the business and the weather too. Asked for the evidence, they can state it, flatly.
Impostor phenomenon: They accept the result but deny it was them: luck, timing, the team. Praise worries them because it raises the bar they'll be caught out against.
Defectiveness/shame schema: Praise brings visible discomfort, close to shame, as if being seen is the danger. They often get more self-critical straight afterwards.
Vulnerable (covert) narcissism: They deflect, then fish: 'it was nothing... did the client actually say that?' The deflection draws more praise out of you.
A next move
Make the praise specific and factual ('you saved the renewal by calling before they called us'), then stop talking. Watch whether they argue with the fact, hand it to others, look ashamed, or come back for more.
If it’s you
When someone praises me, what do I think will happen if I let it in?