Thanks for giving the card name, that helps to investigate possible causes of your problem.
common causes of these problems are full cards memory because the
seller fill the memory with game roms to make the device sell more
attractive, though to save games you need memory and completelly
filling the memory capacity of these cards make them to have issues
like the ones you told before.
The good thing is that you can
connect the NDSL to your PC throught the suplied USB cable and SLOT-2
(GBA game slot) N-Card Flash Linker cartridge.
You can download the required software latest files from the manufacturers website
Once connected, turn on your DS, your PC will recognize it and you will
be able to drag and drop files, then you can delete or copy some less
used game roms or save files from the card to your PC leaving free
space for the others to run.
If nothing of that works for you,
then you would try to find a FAQ or support forum site where you can
find more options to solve this issue.