Depends on why it's not working - if I'm just not feeling it, I'll try re-examining the scene to see whether there's something that needs fixing for it to flow better, or if it's fanwork I might go back to the canon to try and get a better feel for the character voices I need. If I'm feeling it but the words aren't coming together right I'll try writing it differently (brain dump detailed outline, writing it from a different POV than originally planned, dialogue only, etc. - or for a non-writing approach, I might try to gather a soundtrack for it, go for a walk or do something else while thinking the scene over, or something else to give me space to create without sitting in front of a blank word doc) to try and get a better handle. Or sometimes I just need to take a total break and try again later more refreshed.