There is a saying that goes that people are in your life for a "reason, a season or a lifetime" - something like that. Maybe these folk are just busy with their everyday stuff, or maybe it's a sign that that relationship has ended to a certain extent. Who knows, you may pick up the friendship again later. I feel that someone must want to be in your space, and you in theirs. If that want is not there, it's not there. Simple. It is sad though, but part of life. Rather have genuine people in your space than a whole lot of insincere relationships that mean nothing - either to them or to you.