I work several survey only sites in addition to FC and their surveys...and it's the same everywhere you go. Different survey sites attract marketers for different demographics so I've found I tend to do better on some sites vs others.
I also find there's a pattern to the availability of surveys - surveys will start on one site, then if they don't get their quota, I start seeing it show up on other survey router sites. The surveys that start showing up on the other router sites probably have very specific demographic they are looking for and will be harder to qualify for.
I found I tend to have better luck with surveys at the beginning of a month - by mid to late in the month, most of those surveys have the majority of their breadth demographic filled and are again looking for specifics to complete the run of the survey.
The volume of surveys also tend to change up throughout the year. Spring tends to have a small bump as manufacturers are rolling out summer things (vacation surveys, summer activity surveys, clothing, etc). Tax product surveys typically show up around March. Fall tends to be a survey bonanza as you have the back-to-school surveys as well as upcoming holiday topic surveys (travel, food, gifts, etc). I've found January and February tend to be the deadest months for surveys. Late summer (July/Aug) tends to get a little dry too.
If you try surveys during the "bump" periods, you might find you have better luck. Otherwise a technique I use was a 3-strike rule. If I get bounced through 3 sets of router pre-qualification surveys and did not qualify for a survey, I stopped and moved onto the next router.
As far as being disqualified at the end of the survey - that happens elsewhere, not just here. It's the unfortunate nature of some surveys. I've learned often what happens in those cases is the survey is nearing the end of its quota, and happened to reach its quota before you finished. I get a few here and there, and on the survey only sites, if I spend an hour on the survey, I'll complain and see if they can negotiate credit from the group who contracted the survey. Otherwise, I'll just tell myself the situation sucks and move on to something else.