A payment was declined

The payment happens inside their bank app, so the reason is there rather than here. These are the usual ones.

They have never paid this UPI ID before

Most UPI apps hold a first payment to a new address down to a few thousand rupees for the first 24 hours. A large first payment gets refused for this and nothing else. See The first payment limit.

They have hit a daily cap

Most apps stop at one lakh a day across all of somebody's payments, and some banks set a lower one. If they have already paid rent this morning, they may be out of room until tomorrow.

The UPI ID is wrong

If nobody has ever successfully paid you through your page, suspect this first. Test it with a rupee. See Testing with one rupee.

Their bank is having a bad hour

UPI goes down in patches, bank by bank. It usually comes back within the hour. Paying from a different app on a different bank account works when this is the cause.

Their PIN or their account

Wrong PIN, no balance, a frozen account, a card blocked for something else. Their bank tells them which.

What to tell them

Ask them to try a smaller amount from the same app. If that goes through, it was a limit. If it does not, it is the account or the bank rather than your page.

What to do with the claim

If they pressed I have paid and the money is not in your bank, reject it. That is what reject is for and it is not an accusation.