"A book lying idle on a shelf is wasted ammunition. Like money, books must be kept in constant circulation. Lend and borrow to the maximum--of both books and money! But especially books, for books represent infinitely more than money."
I looked at your profile, and I'm impressed - you've only been a member for a month and a half, and you've already registered and released so many books!
I also see that you have several catches, not just one; if you haven't received email alerts for all of them, perhaps there's a problem with your spam filter or something along those lines. You can use the search/sort options on your bookshelf to sort your books by "highest journaler count" to find the books that have been caught and journaled by others, whether you got the emails or not.
Catch-rates do vary quite widely, with 10% a low-average; I get around 15%, and others have reported 20% or higher. But the catches can take time, and as your releases haven't been out there very long yet, many of them may still check in. Some finders don't like to journal books until they've read them (or they think they're supposed to wait until then), and some set the books aside and forget about them for a while, journaling much later to say "found it in the back of the car" or something like that - you never know.
There are things that seem to help with the catch-rate, including making sure that your books are clearly labeled, that you leave them in places where it's easy for people to stop and look at them, and that you try a variety of places. For more ideas, see the Release Techniques forum, which has lots of discussions on this sort of thing. (Oh, wait - I recall you did post there; yours was the books-on-the-fence plan, right? That may not have worked out as you hoped, but if those books wind up in charity shops someone may journal them someday.)
And you might check the Site Watch forum as well; you can see posts about recently-caught books, some of them caught very soon after release and others caught months or years later. [The current record's nearly 10 years from release to catch!] That may also give you some ideas about release methods and locations.
Hope that helps, and that you're enjoying BookCrossing!
Don't really have more to add to Gory's excellent explanation. What I find help w the catch rate is: 1. including a bookmark w more explanation of what BookCrossing is, 2. besides the BC label on the front inside cover or first page, mark it on several other pages as well (without being annoying). eg. on the last page I like to put (use sticker, stamp or handwritten) "now you've finished reading the book, how do you like it? Let us know at BC.com 123-4567890"