Why doesnt this horse trot? 2?

Brief：
Heres the rest to my first question part:Okay, heres what has happened before:She first started doing this with just Karly, me, and my friend Bridget... 

Here's the rest to my first question part:

Okay, here's what has happened before:

She first started doing this with just Karly, me, and my friend Bridget.

Then she started doing it with the owner, who has lots of riding experience but has been using a western saddle and different bridle on her.

They've had professional vets and trainers come out to watch her being ridden, and check her over, but they have never watched anyone ride her English, just Western. I don't know if that makes any difference.

Another thing: It's obvious her saddle (which my friend owns) doesn't fit, but my friend and her parents obviously aren't interested in changing that problem... but I don't think it's that. Because she used to trot with that saddle, for a few months. But of course they weren't riding her all that much then, but now she's doing it with everyone: Western, English, or Bareback.

Have any advice?

Hi the saddle has hurt her back she needs pysio god luck with her! If the saddle fits badly enough that it's blatantly obvious, it's got to go. A poorly-fitting saddle can cause all kinds of chiropractic and other problems. What I'd do: first have a knowledgeable person take a close look at the saddle fit, and get a new saddle that really does fit. Then have an equine masseuse or chiropractor out to look at the horse. Some of them charge a lot, but they're worth the expense. The horse is trying to tell you it is in pain and needs something comfortable on its back. First off, wear some shoes 2 sizes small and lets talk about what poor saddle fitting does.