Mare backs up when asked to go forward

The vet did an ultrasound and didn't find anything.  She took some blood and is going to research which tests to run to see what she can see.  Didn't hear anything yet.
Ziggy is very attentive to this mare.  The mare is even more submissive than I've ever seen in a horse.  Ziggy is acting like a lead mare but what was really distressing to me was that Ziggy wouldn't have anything to do with me or Gerald.  She'd come up to us but then leave right away.  It took us for forever to "catch" her.  That is so unlike her or any of our horses.  Ziggy has always been the first to greet us.

I get the idea that maybe Ziggy wasn't as friendly as she was looking for an opportunity to be leader over us.  I don't know what to do now to "catch" her if she doesn't want to be caught.  Like I said, this is completely new to me.  I get how she'd rather be with another horse than with me.  She's a horse, not a dog.  But how to "talk" to her now?  The pen where she's being kept has a buncha trees in it and isn't suitable to making her move out if she doesn't come to me, you know the old "make the right thing easy and the wrong thing difficult".

Thanks Terry.  Yes, Ziggers is still at the trainer.  They haven't figured out about the mounting.  Personally I think its behavioral.  I think horses at boarding barns have less opportunity to exhibit such behavior when they're isolated in pens rather than all together in a bigger herd in a large pasture/field.  Because that kind of show of dominance isn't unusual at all with dogs or cattle, never mind people!

Anyway, I did mention it to the BO and she explained that it was a herd dynamics things.  But since I'm not there on a regular basis I can't do much about it.  So I'll have to let go of yet another "want" on my list.

Interestingly Penny who used to be so stand offish and more concerned with moving up in the herd dynamic has really calmed down this past year.  Maybe some of it was Buster passing away, he was the lead horse and ruled with an iron fist. I think another part of it is learning that we can be trusted to take good care of her and keep her safe.  She came from a place where she had to be lead mare to keep her and her sister Peaches safe because her owner was even less confident that me.  Now Penny is an aggressive horse, which puts me out and keeps me on my toes. I don't let her snuffle me for fear she'll bite me.  She has bitten in response to being cinched. I think I've got her over that but I don't trust again readily.

What are some techniques to figure out why a horse is hard to catch?  And then what to do about it?
What can I do to convince myself that Penny isn't going to bite me?  How do you regain your trust in your horse again?

Just to add my farrier has started giving her mare papya pills to help with ulcers and nervy tums:)