Checklist:
- SD formatted FAT32 (or FAT16 for non-HC SDs with max 2GB)?
- Use guiformat on modern Windows for formatting 64GB or larger (SDXC).
- Any other operating system than modern Windows should support formatting to FAT32, 32KB cluster size out of the box.
- SD verified with h2testw, F3 or F3X?
- Contacts clean:
- Tried different SDs?
- Micro-SD in adapters can cause trouble even if working flawlessly on PC
- Full-sized SD is better. At least try different adapters for µSDs
If everything fails (reader bad) get a new reader. The repair is not very hard. Refer to ifixit for instructions
Good luck and have fun!