David is right it could be a lot of reasons. Perhaps you need to reword your ad or put more details in, but many times it is for the reasons already mentioned when someone declines a job.
I don't do fixed rate jobs, and generally I have very limited time, so if I do make myself available it is for short jobs or a few extra hours a week as my schedule is already very busy, so I get kind of picky about extra work I pick up.
I am sure it has nothing to do with you, but perhaps you need more details or to redo your ad.
Good Luck Christos.