My mare keeps acting like she doesn't know how to jump. She's a three foot horse so I know she can. Any tips? I bought her a few months ago.

If she isn't jumping I suggest packing her feet cause she could be sore and using liniment also if she doesn't jump ask a trainer to jump her or free jump her
This is just a genuine question, if she used to jump and isn't now. Have you thought about getting her checked over ?
You can start to try to go back to the basics and get her used to it all again or depending on where she came from and what she did.. she might not bother with the little jumps so she doesn't like to do them.. I have a gelding that kept refusing small jumps and then I put them up a bit and jumped over them fine ..
I would take it back to basics, my horse gets a bit defiant every now and then and going back to some light work usually helps us. Just try something like lunging over ground poles or jumping over some smaller jumps.
If she doesn't know how to jump, show her what jumping is in baby steps. Like making her go over some cavalettis or baby jumps (On lunge line or freely.)

If that's not the case and she's just acting rebellious (as I call it). My horse would do the same things occasionally and I would jump him over baby jumps and cavalettis.

Hope this helps.
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