Apple Pay not automatically popping up at checkout

Pay with your iPhone

1. To use your default card:

• • If your iPhone has Face ID, double-click the side button. If prompted, authenticate with Face ID or enter your passcode to open Apple Wallet.
  • If your iPhone has Touch ID, double-click the Home button.

1. To use a different card, tap your default card to see your other cards. Tap a new card and authenticate.
2. Hold the top of your iPhone near the contactless reader until Done and a checkmark appear on the display.

Make purchases using Apple Pay - Apple Support

Are you referring to Express Transit, like riding a subway, or checking out at a grocery store?