They say they've paid but it isn't showing
First, which way did they pay?
By card, through the pay button on the invoice, booking or proposal.
Manually, by transfer or cash, following the instructions on one of your own payment methods.
The answer is different for each, so ask before investigating.
If they paid manually
This is almost always the explanation, and nothing is wrong.
A manual payment is not recorded until you confirm it. The client has paid, and from their side it is done. The invoice waits for you.
Check the payments waiting on you. Go to Home and look under Needs attention for N payments awaiting review. That row opens the list. If the client pressed I've made this payment, their claim is sitting there with whatever proof they attached.
Check the money has actually arrived in your account, then open the claim and Record payment. See Reviewing payment proof.
If nothing is waiting there, they paid without telling CoreHue. Check the money arrived, then record the payment yourself.
Two things that explain the rest of these conversations:
Confirming does not email them. The invoice updates and the receipt exists, but nothing lands in their inbox unless you send it. If they are waiting to hear, tell them. Rejecting a claim does email them, with your reason and a Resubmit proof button.
One claim at a time. While a claim is waiting on you, every pay button on that invoice is hidden and the page reads Awaiting confirmation. The client cannot take their claim back. So a client saying "it will not let me pay" usually means one of their claims is waiting for you.
If they paid by card
Card payments record themselves within seconds, so an unpaid invoice usually means the payment did not complete.
Check the payments list first, in case it is recorded and you are looking at an old page. Filter Payment channel to Card (Stripe) to narrow it down.
Ask them what they saw. A confirmation screen means it went through. Closing the tab at the last step, or a card being declined, means it did not. Clients often remember starting a payment as having made one.
Check their bank. If they are certain, ask them to look for the charge. A pending authorisation that never completed looks like a payment on a banking app but is not one.
Check Stripe if you need to be sure. Anything genuinely taken is there.
If money left their account but CoreHue shows nothing
This is rare, and worth being careful about. Ask them for the reference and check Stripe directly.
Do not record a payment you cannot see. If it turns out they paid twice, or paid someone else, an invoice marked paid on trust is hard to unpick.
Avoiding it
Ask for the invoice number as a payment reference in your manual instructions. Half of all "cannot find your payment" cases are an unlabelled transfer.
Confirm reported payments promptly. Every one is a client who thinks they have paid and is waiting to hear back. While it sits there, they cannot pay anything else on that invoice.
Leave proof of payment on. It is on by default for a manual method. It gives you an image or a PDF to work from instead of a conversation. Switching it off saves the client ten seconds and costs you this whole page.
Say what happens next in your instructions. "I'll confirm within a day" sets the expectation that nothing is instant.