And no, refreshing, restarting, and shutting down doesn’t work. Neither does reconnecting the internet nor trying a different device. They aren’t falling through the terrain, either. In fact, they’re placed above it.
Did you check to see if they were set to not be active on game start, sometimes it sets them to that when you place them. Also if there is multiple sentries in the same spot it likes to remove them if there is “too many”