The tag reads on one side of the phone only
Two different symptoms share this description. Either the phone reads from one region of its back, which is normal and simply means you found the coil. Or the tag reads from one of its own faces, which means it is built with metal or ferrite behind the antenna.
Which one do you have
Turn the tag over and present the same face of the phone. If it now reads, the tag is one sided. If it still reads, only the phone position mattered.
Then keep the tag fixed and move the phone. If reading depends on where the phone touches, that is the phone's coil position.
Both can be true at once, which is what makes this confusing. Test one variable at a time.
The phone side
Every phone has one small NFC coil in one place on its back, and the position varies by manufacturer and model. Upper back, centre, near the camera cluster.
Presenting the wrong part of the phone gives no read at all, which people describe as the tag working on one side.
This is entirely normal. Nothing is wrong with the phone or the tag, and knowing the position converts an unreliable tag into a reliable one.
A case does not move the coil and does add distance, so a thick case makes the usable region feel smaller.
The tag side
An ordinary tag reads from either face, since the field passes through the coil in both directions.
A tag that genuinely reads from one face only is built that way. On metal tags have a ferrite layer behind the antenna, and metal cards have either metal behind it or a window through it. In both cases the reverse face is shielded by design.
This is expected behaviour rather than a defect, and it catches out anyone handed such a card for the first time. About half of them present the wrong side.
A tag stuck to metal behaves the same way for a different reason: the metal is behind it, so only the exposed face works.
What to do about each
For the phone side, find the coil position on the devices that matter and print a target on the tag so people aim at it.
For a one sided tag, mark the reading face. A small printed symbol removes the guesswork for anyone using it.
For a deployment aimed at the public, prefer tags that read from either face wherever the mounting allows it, since every constraint you place on the gesture costs you some proportion of attempts.
And test with someone who has not seen the tag before. They will present it the way your visitors do, which is the only result that matters.
Frequently asked questions
Do all tags read from both faces?
Ordinary ones do. On metal tags and metal cards read from one face by construction, because there is metal or ferrite behind the antenna.
Where is the NFC antenna on my phone?
It varies by model and there is no universal position. Sweep a tag slowly across the back to find it, then remember it.
Does a case change where I should tap?
It does not move the coil and it adds distance, which makes the working region feel smaller and less forgiving.
Is a one sided card faulty?
No. It is how metal cards and on metal tags are built, and marking the reading face is the practical answer.