This is because the ability to visualize--like the disability--is also on a sliding scale as are most things associated with the human brain.
I have extreme difficulty (always have had) visualizing surroundings and describing them, describing faces, after the fact. It takes me forever having to focus on it and usually I have to give up and fake it. Flat out fake it. What I describe most times simply isn't the reality that was "there" when I was there. And who was there to say otherwise? Particularly if I am writing fiction?
Again, as in most things we find ourselves having to do, we accept it and move on, doing the work and leaving the stress behind so that we can do the work.
But I am glad you were able to locate a label for this condition and thereby remove any residual stressors. Now, forward, yes? And especially good work passing the information on to others who may be equally concerned. And opening up this pathway for them and all others to think of all these possibilities of how and why the imagination can be blocked and/or opened.