They tell you that they want to see you, but they are always busy: work, school, family — those are all great reasons. But if they only have time late at night — or certain days of the week without explanation — something is amiss. And oftentimes, they will have plenty of time to see you for sex... just no time to spend before or after.
What to do: Someone who is genuinely into you will make you a priority... beyond just saying the words. And sex is great, but a real relationships needs more than just physical time together.