When it happens
Middle of moderate-stage dementia
Why it happens
Eroding recent memory as well as a distorted sense of time are the culprits. (An hour ago may seem like a year ago; five minutes can feel like five hours.) Memories are inconsistent. The person may ask, "Why don't you visit me?" to someone who visited only yesterday, or that morning. But other events, even seemingly minor ones, are remembered.
What you can do
Chalk these omissions of memory up to the disease; they have nothing to do with the quality of a visit or how strong your relationship is.
Don't argue with the person: "But I was just there!" This only makes them feel unnecessarily bad or defensive.
Respond with reassurance to accusations of not visiting: "Well, I can't wait to see you again!"
If the person expects a visit (not realizing you've just made one) and you can't return soon, try a social white lie: "I'm planning to come real soon; we always have such a nice time."