Help No space on device?

[Cougaram]

I have been using my Samsung S4 to view movies. I've never loaded more than 3gb at one time and after viewing the movie I delete the movie(s) to make space for new ones. However, this weekend, with plenty of space on my device (w/16gb card), I get an error msg saying 'cannot copy' There is insufficient space on the device. It makes no difference whether I try to load the movie onto the sd card or the phone. Same msg.

My phone stats are:
Phone... 4.82 GB free of 9.15 GB
Card.... 5.63 GB free of 14.8 GB

Can anyone shed some light or share some tips?

TIA