I can not see images in my inner eye most of the time, but I do while dreaming, and I have had a single experience of doing so while meditating, which was clear

I can not see images in my inner eye most of the time, but I do while dreaming, and I have had a single experience of doing so while meditating, which was clearer than anything I've experienced. So I have basis for comparison. The difference is sharp and unmistakable. What is frustrating is that my one experience suggests it ought to be possible for me to train my ability to, but no attempts have succeeded (I've not made that many - it was an interesting experience, and I'd enjoy having it again, but it wasn't earthshattering enough to spend a lot of time on)