They keep bailing on you
"Things come up on occasion, but if it becomes a regular pattern that's a problem," Perry said. "In addition, if they cancel at the last minute on a regular basis, they probably have a better offer."
The bottom line is that people will make time for the things that they care about. If someone is serious about wanting to be with you, they won't allow you to slip through the cracks. The sad truth is if you have to question if they care, there's a good chance that they don't.
The less time you waste on someone who leaves you hanging, the more time you'll have to spend with someone who never would.