What are common reasons clean medical claims still get delayed by payers?

We’ve noticed that even when medical claims are submitted as “clean claims,” payment delays still occur frequently across different payers.

Based on internal billing workflow reviews and denial trend analysis shared on https://prombs.com
, issues such as payer-specific edits, post-payment reviews, and documentation mismatches can still slow down reimbursement.

I’d like to understand from others in medical billing:

Why do payers delay payments even on clean claims?

Are certain CPT codes more prone to secondary review?

What proactive steps help reduce AR days despite payer delays?

Any insights from experienced billers or revenue cycle professionals would be appreciated.