Apple Pay works sometimes and doesn't sometimes.

I recommend having Apple Support do remote diagnostics on your iPhone and pay particular attention to the NFC module. If you’re in the US please call 1-800-MY-APPLE or outside the US, consult the support articles below.

Contact Apple for support and service - Apple Support

If you’re near an Apple Retail store you can also have diagnostics performed at the Genius Bar. You can make an appointment at the link below.

Genius Bar Reservation and Apple

Support Options - Apple

Please walk be through exactly what you’re doing step by step. Do you have a couple of examples of merchants it works at and a few it doesn’t work at?

When you double click the side button what happens?

What country are you in? Is the iPhone in a heavy case?