I had this problem with Sierra.
It was hard to train her because we couldn't catch her in the act because she would never jump over the fence when someone was looking.
Unfortunately.... what worked was a shock collar.

I hated every second that thing was on her, but it's been a few years since she's even had it on. It was the remote type.
We'd let her out and watch her from inside the house.

Something you can try would be the tilt in's that fences have on the top.
(in most cases, the tilt in part of the fence has barbwire on it, but you can use chicken wire)

Please know that I am NOT telling you to use a shock collar, I cried a lot when we started using it on her.
(it was mostly my husband’s choice to use the collar.)

If your dog will jump the fence with you around, I'd tie a leash to her and correct her when you see she is thinking about jumping.

If you have a behaviorists or trainer in your area, it might be a good idea to talk to them.