@Fkdel Hi, I just stumbled upon your post, while traversing through the forum (I just arrived here ;-)). I assume you're talking about "mental images". Do you have any sight impairments or similar? This happens often to my gf when we're tripping or meditating. She can't visualize things in many such situations. You can try to access your other senses first - like try with music. Depending on the situation, you may drive yourself into a synesthetic state, where you can "hear" or "feel" your visualizations. That might help at the beginning, later you'll find your way in the sight area as well.
Good luck!