Are you comfortable posting or PMing a link to your contest? I would love to see the results. I just finished one on 99designs and had pretty good entries.Right now, I'm using elogocontest which is similar to 99designs.
I think it really depends on how specific your description is and how easy it is to achieve your specified goal. There were two other contests started within a day of mine, both with much larger prizes ($100 compared to my $45) but I've gotten more submissions than either of those contests and a lot of them are VERY high quality stuff. The only difference I can see is that I was somewhat specific in my design brief, and I'm also very specific in the comments I leave for each design, leaving designers the ability to tweak it to be more like what I want.