I have a 16 yr old Arab mare who is a very inconsistent "trailer-er." Allow me to explain. For a long time, getting her on the trailer could take HOURS of your time. I have worked with her extensively on this, and for a year had her self loading. After that we lost access to a trailer that we could work with consistently and she regressed quite a bit. Some days she's a peach, other days she's a...witch or something like that ;)
At this point I have her loading regularly with a few issues: issue 1. She will load up and then shoot straight out of the trailer before anyone has enough time to latch her in or clip her halter. Issue 2: if you have a second person around, she refuses to load. She "sees the trick coming" and does NOT want to be latched in there
Interesting fact? She is perfectly sane and calm once she's latched in. No issues while in transport at all.
While I appreciate all advice and opinions, please save yourself the trouble if you have any food motivated tips. She simply does not care about oats, hay, her favorite treats, etc. Sometimes a peppermint can work to stall her from backing up about a second longer once already loaded, but clearly this is not a solution, just a patch job.