Thank you for all of the above responses.
"Ancestry.com has not uploaded findagrave.com indexes for all states/places."
Some of the items that I am referring to are found in the same cemetery next to people who are found through the ancestry search, so it is not necessarily a state/place or even a particular cemetery that is not searchable. Sometimes they are spouses or children in the same plot or even on the same pillar or marker.
I have been trying to determine why some make it into the ancestry search and why some don't. I can't tell why. At first, I thought the ancestry search was only locating ones with photos but it was not that. I have also ruled out that it is a problem with particular cemeteries or particular posters. Nor does it seem to matter whether they have been on findagrave a long time or for a somewhat shorter period. It does seem that the newest ones frequently do not appear but this doesn't provide an answer for many of them.
I would suggest to others that after you locate someone with an ancestry search, go to the findagrave site and search that site directly for that cemetery. This is also important so that you can verify frostfreedet's other valid points.
I do realize that findagrave is not a static database and I am guessing that this may have something to do with the issue particularly for the newer items. There does appear to be something more than that but I don't have the answers.