It is very possible that there is not enough space on the SD card for what you are trying to transfer to it. You need to find out for sure how much space is currently available.
If the card has a lot of available space on it, then SD card may be dying. It happens. They wear out after a while. In this case you would need to back up what you have on the card (move it to the phone or copy over to a computer) and replace it with a new SD Card. I would also suggest doing this soon, as over time, more and more of the card will not be accessible, and you could lose things you already have on it.