I also have friends with whom I will schedule a get-together with, not hear from for days, and then need to text to say "are we still on for tonight?" only to receive an "OMG! Something came up! Can we raincheck?" reply.
Some people reschedule coffee dates – the simple affairs that take up all of 30 minutes – several times before you nail them down. Some just don't turn up and say they forgot. In many cases, you end up wondering if you should call it quits on that friend.
With constant re-schedulers, are they are trying to send us a message? Are we not that important to them? Is our relationship not that close?
I often worry that these re-schedulers are passively-aggressively trying to phase us out of their lives. Why? Because it's not that hard to see people you want to see. I don't understand the concept of the "something came up" excuse. If it's that important, a full explanation can be given. "Something came up" sounds like you have diarrhoea – which is a perfectly acceptable excuse for rescheduling. I wouldn't judge you for telling me honestly.
So how should we deal with constant re-schedulers? I believe in giving everybody a bit of lenience, but only to a point. Four rescheduled coffee dates is enough.
My first tip is not to give a re-scheduler too much priority. Don't organise an entire day around them. Always have other stuff to do so you're not left bored and alone if/when they want to change the time and day yet again. They are allowed the same gravitas as they give you, not more.
You also much be polite to them when they change your plans on you. As amazing as it would be to shoot back an "are you serious?!" message with an angry-face emoji, to not-so-subtly tell them that they have wasted your time (yet again), don't do that. You can't guilt people into keeping plans with you. If they think you're needy they'll also want to see you less, not more.
I like to be casual and nonchalant with re-schedulers, even when I feel the opposite inside. Perhaps I feel aggrieved, but I will tell them not to worry. When possible, if a re-scheduler has made you angry, it pays to be understanding.
I try to accept, then readjust, my mood so I don't care for very long. The re-scheduler doesn't need to end up with the full wrath of my disappointment in the moment. Sometimes it's a bad idea to say how you think you feel – better to leave it until you have calmed down, at least for a few minutes.
Lastly, I like to leave future plans in the hands of the person in mention. If somebody needs to reschedule, it's up to them to arrange it. I don't like to proactively throw out times and dates that might work. It's the other person's job to put the effort in now.
This tactic also helps in figuring out if they are serious about phasing you out, or if they are generally just encumbered by their own disorganisation.
Sometimes this means you lose touch for a while if it's a friend. In a work situation, it might mean you'll be left to your own devices. I accept both outcomes as part of life. You can't force anyone to spend time with you. You're better off managing without them until they can finally get themselves together.