I've had this happen and it's a tad frustrating when we meet someone with whom we really want to work.

I had this happen a few weeks ago; followed up a couple of times with no response, but then out of the blue, the prospect called, said he had been going to open houses in all of the communities I had mentioned as being possibilities for him and now he had narrowed down to one community and wanted to see whatever was available within certain criteria in that one community. Wow, who doesn't love that? A few days later, he was ratified. So, in his case, he wanted to do his own due diligence without an agent tagging along. I am finding a lot of folks are like that - heck, *I* am like that! LOL
We never know what is going on in other people's lives and I think as long as we're respectful and follow up with something of value, it's fine to keep trying...my approach is to go look at property in which they may have an interest, then contact them with "I just saw a house I think you may like, let me know if you'd like to see it" - I make it casual and try to sound as if it were strictly coincidental, so they don't think I'm out there looking for them specifically. If they don't respond to that, then I figure something else is going on and I touch base periodically, but don't bug 'em. It's amazing how often someone pops up after a few months, even....