It makes sense to me that the visions Harry sees into Voldemort's mind don't happen all the time. In year five he sees into Voldemort's subconscious pretty often. In year six Voldemort starts using Occlumency to shut him out. In year seven the visions start up again. What doesn't make sense to me is the changes in the quality of the visions that start half-way through The Deathly Hallows.
The only other thing that mattered to him was that his scar had begun to prickle again, although he did all he could to hide this fact from the other two. He sought solitude whenever it happened, but was disappointed by what he saw. The visions he and Voldemort were sharing had changed in quality; they had become blurred, shifting as though they were moving in and out of focus. Harry was just able to make out the indistinct features of an object that looked like a skull, and something like a mountain that was more shadow than substance. Used to images sharp as reality, Harry was disconcerted by the change. He was worried that the connection between himself and Voldemort had been damaged, a connection that he both feared and, whatever he had told Hermione, prized. Somehow Harry connected these unsatisfying, vague images with the destruction of his wand, as if it was the blackthorn wand’s fault that he could no longer see into Voldemort’s mind as well as before.
(Deathly Hallows, Chapter 22, The Deathly Hallows).
I find it hard to believe that the changes were because of the wand since he sees into Voldemort's mind perfectly clearly later in the book when his wand is still snapped.
Is it because the connection between him and Voldemort is weakening? That's certainly not what other parts of the books suggest. Dumbledore seems to contradict it as an idea:
“Meanwhile, the connection between them grows ever stronger, a parasitic growth."
(Deathly Hallows, Chapter 33, The Prince's Tale).
I'd also rule out the idea that Voldemort was using Occlumency since he didn't seem to be doing that at all during the events of Deathly Hallows.
Has it ever been explained what the cause of the disruption in the clarity of the visions was? Is there an in-universe explanation?