Interesting. I can edit my posts, and have, but there's a ridiculously short period of time in which I'm able to do so. In other words, I can't come back an hour later and fix a misspelled word.
Now, the Post/Reply boxes will put the squiggly red line underneath words they think aren't spelled correctly. Check that first. Won't fix grammar though.
I also find it helpful to "Go Advanced" when replying as it allows me to see what the post will actually look like.
Hope this helps.
My after-post edit demonstration[/edit]