i have found that over time, the installed apps list with begin to shorten. It seems to drop some apps over time, and I don't know why. I had 30 + items in my installed list only for it to stop sending me updates. When I went to check I had 5 items in my list. I have had it happen a few times now.
I don't know what other info would be needed for this. Please let me know if you need logs, more info, etc.