Make sure the MSR is securely plugged in using the correct sleeve/adapter.
Make sure there is room around the MSR for full swipe movement. Don't crowd the workspace. Allow plenty of space for full card swipe.
Check to avoid any magnetic interference from other devices.
Make sure to swipe the card with the magnetic stripe facing the magnetic read head. If the POS device screen is facing toward you, the magnetic stripe needs to be held with the stripe away from you.
Check the card's magnetic stripe for damage.
Don't swipe too fast. Don't swipe too slow.
Swipe in a flat movement. Do not lift up or push down.
If the MSR appears to have gone offline or to "sleep", tap the Tap here if reader isn't working button on the Checkout screen.