Every damn day! On most "swipe" terminals you should be able to get it to work by just placing the phone on top but if that doesn't do it try holding the phone sideways and flat to the slider (tapping the side of the slider holding your oh one as if it were a card, if that makes sense). Also make sure you don't have a case that might interfere with the transmitter (which is pretty much around the center of the phone).
If you can't get it to work still, here are some troubleshooting options:
1) Make sure Samsung Pay is set as default (or to use the currently open app) in the Tap to Pay settings on your phone.
2)Uninstall / Reinstall Samsung Pay.
3) Do you have issues with wireless charging? If so that might mean you have damaged hardware and will need to repair/replace the phone.