I've been in this boat many times myself on this site and on others. I've had surveys pre-qualify me with demographic info, let me in to take and complete the entire survey then ask the same demographic questions at the end of the survey and disqualify me after I've already the entire survey. The demographics were fine to let me in and get my answers but not fine to pay me for them? I've had surveys stop working/hang up on me halfway through. I had one survey that asked me to input the date when I purchased something in the MM/YYYY format; the two digit month in one box and four digit year in another. When I typed in 10 in the first box and 2014 in the second, it kept telling me I needed to use the MM/YYYY format. And then there is always one of my favorites: make it 85% of the way through a survey and get told "we've reached max capacity" or "we have enough responses from those who meet your demographic". Why not tell me that up front and save me 40 minutes?
Survey issues like these have happened to me at least four or five times in the past couple of months on another site I use. I took screen shots of the surveys that failed (stopped responding, didn't process input data correctly). I contacted customer support on that site and explained the problem for the surveys that malfunctioned and the ones that disqualified me. I told CS what the surveys were about, how long they took and gave them as much identifying information as possible. On every single occasion that site paid me the full amount of the survey and apologized for the trouble. Different sites handle things like this in different ways.
I completely agree with ccandpam. These types of surveys are the kinds I call "data miners". They let you in, get your time and answers then cut you off after they have everything. Some survey companies are worse about that than others. The longer I do the online sites like these the more I'm learning which survey companies I can feel relatively comfortable with and those I don't. Regardless, I never feel truly comfortable with any online survey until I see that money go into my account and until enough time has passed to where I know they won't take it away because I answered a question 'no' that they feel I should have said 'yes' to or because I they didn't like my demographic.