The tapping functionality of Linq uses NFC technology, which works with nearly all Android phones and iPhones 7 and newer.
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A few essential points that might be related to the problem you're experiencing:

1) Metal phone cases can interfere with the NFC signal. All non-metal cases work fine.

2) The receiving phone must be unlocked and awake for your Linq product to work

3) If someone has an iPhone XS/XR or newer, the NFC is always on, and the notification should appear toward the top of their phone

4) If someone has an older iPhone but has the newest iOS installed, they can pull up the NFC reader in the Control Center and then tap your product.

5) If someone has a phone without NFC capability, they can open the camera and point it at the QR code on your Linq product.

6) Some Androids may need to turn on NFC in Bluetooth & device connection within device settings.
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PS. For iPhones, the NFC module is near the top 1/4 of the device (front or the back), so you'll want to bring your Linq product up to that spot. For Androids, it is usually in the middle or further down on the phone. It is vital to line up the reader with the chip inside the product.
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Here is a helpful pdf for turning on Android NFC: https://www.mtroyal.ca/AboutMountRoyal/TransportationParking/Permits/Android-How-To-Turn-On-and-Use-NFC-1.pdf