This is a sensitive subject for me because when my thoroughbred was 6 (now 20) we had gone to a show and he did not want to load to come home. I had lots of helpers (from that stable) and they were not nice helpers. To make a long story short, After my parents telling them to leave the area and let us handle it, it took us 6 hours to get him back on the trailer and to this day, he WILL NOT load easy because of what they had done to him mentally. It involved very similar activities that were used with Tessie, ropes, smacking, yelling, pulling and all this yee-hahing. If you are not careful, you will mess them up for life. I didn't care how long it took to get him in there, as long as I got him in there safely and kindly.
I had a QH that wouldn't load when we were out on a trail ride and just like katrinag, I rode him home. He usually loaded right up but for some reason that day he said noo way! Another trail rider that was there (that I did not know) suggested whacking his back end with a shovel
Some people will try anything to get a horse to load and not think twice about the effect it will have on the poor animal.
I would definitely ask her previous owner what kind of trailer she is used to being trailered on and try that. Some horses are just picky. I have had many that would only get in ones with ramps and other who would only step up into with no ramp.
I had a QH that wouldn't load when we were out on a trail ride and just like katrinag, I rode him home. He usually loaded right up but for some reason that day he said noo way! Another trail rider that was there (that I did not know) suggested whacking his back end with a shovel

I would definitely ask her previous owner what kind of trailer she is used to being trailered on and try that. Some horses are just picky. I have had many that would only get in ones with ramps and other who would only step up into with no ramp.