I cannot find the right spot on my phone
A slow sweep with the largest tag you own finds it in under a minute. Move at a crawl rather than tapping repeatedly, since a fast pass can cross the coil without staying long enough to complete a read. Then mark the spot, because you will forget it.
The method
Take the largest tag you have. A card is ideal, because a big antenna is easier to find with than a small one.
Remove the case. You are looking for the coil, not testing the deployment, so eliminate what is in the way.
Lay the tag flat on a table and move the phone across it slowly, back down, starting at the top and working down in strips. Take several seconds per pass.
When it reads, stop and note where the tag sat under the phone. Confirm by lifting away and returning to that exact spot twice.
Why tapping repeatedly fails
A read is not instantaneous. The tag has to power up, be selected, and transfer its content, and a fast pass can cross the coil without giving it that time.
So tapping quickly in several places can miss a coil the phone would have found with a slower movement, which sends you looking in the wrong region.
Repeated tapping also changes two things at once, the position and the duration, so a failure tells you nothing about either.
One slow sweep beats twenty taps.
Mark it once you know
Put a small mark on the case, or remember it against a visible feature such as the camera or the logo.
Write it down if you are supporting several devices. A short note of which model has its coil where saves the whole exercise next time.
Retest after changing the case, since a thicker one shrinks the usable region even though the coil has not moved.
Doing it for other people
For a deployment, you cannot ask visitors to do this. Design so they do not have to.
Use a large tag, which forgives an approximate approach, and print a target so people aim at something.
Say what to do in a few words on the tag. "Hold your phone here" converts a wave into a touch.
And accept a proportion of failures on first contact. The design goal is that the second attempt always works, not that the first one always does.
Frequently asked questions
Why does a large tag help me find it?
Because it covers more area, so the coil is easier to intersect. Once you know the position, small tags work fine at that spot.
Should I take the case off to search?
Yes, while searching. Put it back afterwards and retest, since a thick case shrinks the region without moving the coil.
Does the position change with software updates?
No, it is physical. What can change is whether NFC is enabled and how the phone behaves in the background.
Do two phones of the same model have the same spot?
Yes, since the hardware is the same. Different cases and accessories on each are what make them behave differently.