QR code does not open a UPI app

First work out which code was scanned, because the two behave differently and only one of them is supposed to open a payment app.

The code on a poster or a share card

That one holds a web address. It opens your Kulhad page in a browser, which is what it is meant to do. From there they pick an amount and pay.

If it opened your page, nothing is wrong.

The counter code

That one holds a payment. It should open their UPI app with your ID in it.

If it does not, work through these.

• They scanned it with a plain camera app rather than inside a UPI app. Some camera apps do not know what to do with a payment address. Ask them to scan from inside Google Pay, PhonePe or Paytm.
• They have no UPI app installed. Nothing to open.
• They are on a computer. A payment address only opens on a phone. On a desktop, the page shows the QR for them to scan with a phone.

If it will not scan at all

• Print at 2.5cm across or larger.
• Leave clear white space on all four sides.
• Keep it black on white.
• Matte, not gloss. Lamination and shiny paper throw glare under a shop light, which is the commonest reason a code on a counter needs three tries.
• Check it is not scuffed or folded across the middle.

Checking what a code actually contains

Kulhad has a free tool that decodes a QR and tells you which UPI ID it pays, without paying anything. It is at Check a UPI QR code. Worth doing on any printed code you did not stick up yourself.