I notice that many of the no-picture wishlist-entries don't show an ISBN; that might have something to do with it. If those entries displayed covers before, then something in the recent site tweaks and/or a change in the Amazon cover-lookup process could be the cause. I do know that there had to be some site tweaks to correct the cover-lookup problem at registration; maybe the wishlist covers disappearing is a side effect of that?
Re the absence of ISBNs on individual wishlist entries: this can happen if you entered a wish manually and did not fill in the ISBN field, or if you used the global-overview page's "add to wishlist" button if that page had no ISBN. If you didn't use either of those methods when creating the wishes, or if you're sure that your no-cover wishes did have ISBNs at some point, then I don't know what's going on there.