Perhaps you only had a list of attendees or their business card had limited information, so you try to track them down via Google or on Linked In or Twitter.
Some people just don't want to be found. They've hidden their email address on LinkedIn, so you can't email them that way and if they've restricted access to their LinkedIn profile, you'll need an email address in order to connect with them. They don't have a profile photo, so you can't find them that way.
You may track down their website but many times it doesn't have either an email address or a name, so you're not sure if you've got the right person.
Make it easy for people to contact and connect with you - after all if you have an online presence, whether through your website or a social media profile you presumably want to found that way.
You'll find me on LinkedIn and Twitter, Like my page on Facebook or you can contact me directly or use the form on my website. Why not book an initial chat with me via Tungle.me.