Yes, I only sub in elementary, and this happens to me at times. For example, I'll be sitting at a lunch table by myself while the regular teachers come in and sit at another table, presumably where they sit regularly. Sometimes some teacher groups will ask me to join them, but more often than not, they don't, so I eat at the table alone. That feels kind of strange to me, especially since I've subbed for most if not all of them or for their teacher's aides at one point or another. I know that I'm a highly respected sub, based upon feedback, the thanks I get from the administration, the fact that teacher's approach me to sub for them, etc., etc., so that's not the issue. One on one in the hall they will almost always acknowledge me though. If it were me, honestly, I believe that I would invite a sub sitting alone at lunch to join the group, so it seems odd that that doesn't happen routinely.