The SD card slot of the Nokia might be damaged and fails to detect the SD card. Or, the micro SD card contacts are worn and torn after two years' use and make it hard to be recognized by the phone. It's also possible the card reader is making contact with your micro SD card in different places from the internal slot of the phone ones is. One last chance is you've reached the limited read-write chances of the SD card.
A damaged, broken, dusty, and wobbly SD card slot can cause the micro SD card not recognized by your phone. As you've said, the SD card works perfectly on a Windows computer or with an external SD card reader, so it's highly possible the internal SD card slot of your phone is faulty.
The micro SD card has many dents on its surface. If you've frequently used the micro SD card on various devices during these two years, the contacts of it are no longer delicate as before.
The external SD card reader connects to your micro SD card with different places than the internal one does. Although it should be slight differences, sometimes the minor touches make things different.
Possibly, you've run out of the reads and writes of this micro SD card. An SD card does not last forever and you may reach the limited chances of read-write.
One more chance is the disk driver is outdated and fails to recognize the micro SD card. You can update your Android system to enjoy the latest driver.