I was on Stack Overflow just now editing someone else's question. When I submitted it, there was a queue on the edit, so my edit didn't go through. So I went back to leave the editing process. By doing that, I can't edit anyone's work. I finding that really annoying.
As Shadow Wizard said:
Hover over the gray edit button to see why you can't edit.
It's not that it's not working or we'll all notice that issue at some point. It is probably one of two reasons with the latter being highly unlikely if you have a low edit rejection rate:
The post has a pending (suggested) edit that still needs to be reviewed
You have an editing ban
The first reason is probably the more likely reason here of course if you can edit other posts but just not that one.