There are a few reasons why it might not work :

The card
1// Is your Smappee card is activated for public use ?
You can check this when looking in your Smappee app -> Settings -> Wallet
When you see the RFID-token listed, you card is activated for public use.

2// Deactivated or expired ?
The company or system administrator has turned off your specific card, or it has expired.

3// When did you activate your Smappee card for public use ?
Have you only recently activated your card ?
Using the card on a Smappee charger, it should work immediately.
With the same roaming partners, this can take up to 2 days.
With different roaming partners, this can take up to 1 week or slightly longer.

4// Do you keep your card in your phone case , or in the center console charging pad in your car ?
It messes with the electromagnetic interference, which can burn out the RFID-chip.
Every card contains an RFID chip that must connect with the station’s RFID reader.
If the card is defective, it will not work anywhere—including Smappee stations.

The charging station
1// Is the charging station listed in the Smappee app as compatible ?
Where can I use my Smappee charging card and how much does it cost? – Support

2// Charging station issue ?
If your card works at a Smappee charging station but not elsewhere, the problem lies with the other station (e.g., poor internet, broken RFID reader).

3// Reader malfunction ?
Sometimes, the problem may not lie with the card itself but with the RFID-reader in the charging station.

Human interaction
1// Are you swiping your Smappee card correctly ?
Incorrect card placement can cause issues.

2// Metal blocks the signal.
Make sure you swipe the card in the correct way

3// Distance/Angle: 
You are holding it too far away, or at a sharp angle? RFID cards need to be parallel to the reader for the best signal.

What should I do first?

The card
Is your Smappee card activated for public use ? 
-> no  -> activate your card !
How do I order and activate a smappee charging card as a private individual ? – Support

Deactivated of expired ?
Yes -> check with your company or administrator.

When did you activate your Smappee card for public use ?
Try using your Smappee card on a Smappee charging station, this should work immediately, other stations might take a few days.

Do you keep your card in your phone case , or in the center console charging pad in your car ?
Use your Smappee card on a Smappee charging station, if this does not work, your need to order a new card.
How do I order and activate a smappee charging card as a private individual ? – Support
How do I order and activate a Smappee Charge Card for charging publicly? – Support

The charging station
Is the charging station listed in the Smappee app as compatible ?
Where can I use my Smappee charging card and how much does it cost? – Support
No ?  If the station is not listed, your card won’t work there.
Use another station shown in the app, or pay by debit/credit card (required by AFIR legislation).
 

Is the charging station listed in the Smappee app as compatible ?
Where can I use my Smappee charging card and how much does it cost? – Support
Yes ?  If your card works at a Smappee charging station but not elsewhere, the problem lies with the other station (e.g, poor internet, RFID reader malfuntion).
Contact the charging station operator directly  (the details are on the charger).
Start the session via the use of debit/credit card ( (required by AFIR legislation).

Human interaction
Are you swiping your Smappee card correctly ?
How and where to swipe a RFID card on a Smappee charging station – Support