There are , of course, a miriad of potential reasons a cow shows up OPEN. Believe it or not, MOST of the every day reasons are the small ones, not the huge string of metritis hanging out etc. It may just be HER timing was wrong, or stress (HOT) or a very small amount of low grade infection. Alot of things.
The best way to find out is of course to go and try to flush her. Make certain she is in fact open. Bring her in and infuse her with a good intra-uterin infusion product (I use Bio-Mycin C), wait for her to have a NATURAL heat. Set up her sequence on that heat -=-most of these work out real well. If there are tons of scar tissue in there, it gets less productive.
Give her a try -- if she has proven herself, the cost is small compared to the possible benefits later. I'd bet ya she breeds after you flush her too -- most of them will!
Terry