10 days is a lot and could be a problem.

Payment not guaranteed is a problem.

About the 10 days deadline,

first of all, a shorter deadline will get the best designers working for you as soon as possible, a long deadline would just make them postpone again and again until they forget about it.

Second, designs will come in a lot quicker when the deadline is closing in.

And third, a week from now, we would hardly find your contest, it would be on the 10th page or something like that, that can't be good.

You have some valid points there, however I do intend to leave this running a few days to see if this is indeed an issue and I am not getting the responses.

I have got some good concepts thus far which I will be responding to this weekend.