Why do girls add me on snap, snap me, then just stop?

So this has happened to me with 6 or so girls and I feel like there’s something I should be getting or something they want but I don’t really know. So a girl would add me on snap and sometimes it would be someone I wouldn’t know, but goes to my school or would be someone I do kind of know, so I would add them and either one of us would just start snapping each other for a couple of weeks and it usually didn’t include face pics. So then out of nowhere they would usually just stop snapping me. So I feel like a dumbass because I feel like there’s something I should be knowing what to do.