How about I comment from a prospective of hiring someone on these sites. I posted a few jobs I was looking to hire for and couldn't believe the responses.

So, looking at it from the other side the person seeking is "testing" the waters trying to find someone good to work with (for future work down the road). I'm willing to pay more LATER once I see how their work is and their follow through. I've been burned on these sites too from the hiring aspect when I lost my deposit where the worker hired didn't follow directions and continually made mistakes so I cancelled the job, resulting in loosing my $.
I would say these sites are an excellent place to start and build your clientele. Make sure you have samples of your work and before & afters (with approval from your client if it's a true piece of work). Furthermore, I would receive offers to do the work and then when I replied with a question, never heard back from the applicant. I also do search by the USA only when seeking applicants...my preferred choice (sometime there are very fewer applicants for certain projects by doing this). If you don't have any ratings, you probably won't be included in my search. I typically want people with a few ratings this way I know they've done work on these sites before. So, if you're one of those who's not getting any work off these sites, that could be another reason too. Keep in mind your keywords of what you offer. There are times I search for a person to do a project and few appear because the item isn't in their list of duties and such.
You can tell the people who are successful on these sites. Look at other profiles and how much background they give about themselves and what they offer. As a clientele is built your rates can go up. There have been several people I've contacted privately about a project because I see their following and good ratings. If you don't have any jobs or projects on file I might not be willing to test the waters with you (sort of speak). I agree with a few others here that you should build your following, ratings and clientele if you want to be successful on these sites.
BTW, I've found excellent GIGS on Fiverr. Keep in mind you can offer additional "tasks" for more than $5 buck! So, my header designer had hers set up as an example, $5 for a header with three designs and NO changes. If you want a 24 hour turn around with unlimited changes it's an additional $25 GIG. So, it's all in how you work it! But, I will tell you I didn't want to spend a ton of money testing the waters since you never know what you're getting. I've also had a few poor Fiverr jobs but felt for the money testing the waters I didn't feel so bad.
I've had the same issue with a job posting I did here on the VAF's. The work was substandard and I felt I wasted good money for a poor job. What recourse did I have? None...had I been able to test the VA with one of these sites, I would have not used her again especially for the project I hired her for. I guess it's a catch-22 both ways.
Hope this helps!