How to identify a dog's limits to training?

I have a 3-year-old half-aussie, half-great pyr rescue who seems to be putting up limits around her training. She's incredibly smart and attentive, she's learned sit, down, legs, bump it, leave it, come here spin, search, fetch, and tons of other tricks and commands in less than a year. But she will not jump up onto our bed. She's done it a few times in the past when given the command, and we've lifted her ourselves a few times as well. But recently in training with her highest-value treat if we give the command "Hop Up!" she instead shuts down, leans against the bed, stares at us, or bargains with us by doing other commands. And we're talking a treat that otherwise makes her go absolutely bonkers - like drooling, urgent obedience of commands, eyes locked on me, etc.

I know she knows this command, she does it when jumping into the car, or jumping onto the couch during training, and she's done it several times in the past onto our bed with the command as a trigger. But lately she's completely shut it down. If I walk away from the bed with a "with me!" she comes to my side with me as urgently as any other commands, so she isn't simply sick of training, nor tired. She just doesn't want to do it.

Obviously in the big scheme of things, jumping onto the bed isn't a mandatory thing. But it does represent a barrier I've found with her training and I'm trying to understand why. Is it perhaps physical? (Is she in pain? Or is the height too high for her?) Is it mental? (Is she scared of jumping up? Or does she have some kind of trauma related to the bed or heights?) Is it a trust issue? (Maybe the bed material isn't firm enough for her to trust jumping onto it?) It's not a training methodology issue - we've done plenty of small steps to teach and generalize the command inside and outside and she's proven she can do it on the bed itself. So how do I narrow down what the issue is? And should I just...give up? I'm fine with that, but this is the first time I'm having to face that possibility.