SD-Card can't write to

Sometimes, when I want to copy files to the SD-card, Dolphin reports that it can't write on the card. I verified the write-protection, but that doesn't help. After as few umount and mounts, I formatted the card as it was in vfat, and then, the card works again. I could write on it.
But after a few mount's, I can't write to the card again. I formatted it and things go fine.

Is this normal with this kind of card? CF card simply works fine.