Changing images around in galleries

I’m having trouble with moving images around after uploading them into galleries. Whenever I try to move an image I get a weird blue streak and the image won’t move. I used to be able to just grab an image and move it into any order I wanted but now when i try to grab an image nothing moves. Why is it doing this? Why can I not do what I used to be able to do and what does the blue streak mean?

Any help much appreciated.

Thanks!