Allow me to be a bit judgmental here

 Three strikes is a good rule of thumb I think, the fourth one doesn't work and the batter goes home.

It is important to give people benefit of doubt, be mindful of the fact that they might have a lot going on in their lives, and surely even the best of us have troubles brewing now and then. Being kind and empathetic helps us connect to our fellow beings better, understand their pain and the circumstances that lead to their decisions.

So when you reach out to them on several occasions, with nothing but the best of wishes and excitement and eagerness there's hope of a semblance of reciprocation. The responses don't have to match the energy level, or be in the same frequency even. A curtsy reply, merely as a part of a social convention, an acknowledgment that we're all members of the human society and as human beings we've internalized some manners we're expected to keep is unfortunate though perfectly acceptable. Not everybody needs to be friends, friendships can decay and wither away. Dynamism is a fact of life, things change, people evolve, circumstances develop. With all that in mind, if there's three real attempts to make sure things are a-o-k, and no response, despite the fact that the counterparty appears to have way happier memories of former interactions than oneself, the contradictions lead one to worry about the general well-being of various parties, and also consider perhaps one's making fool of oneself and one needs to permanently disengage from an ongoing situation.

Patience is a enviable virtue, infinite patience is godly. We are however only mere mortals.