It's most likely because you don't 'fit the criteria', as they usually say. Sometimes you have to 'predict' what they are looking for (I usually keep the age and location, but make some stuff up). One thing you should do though is:
1. If they ask if you or anyone you know always work in a particular area, say 'None of the above'. What I have seen is if you answer one that is linked to the topic of a survey (eg. working in banking for a survey about bank accounts), you will not qualify for the survey.
2. If they ask if you have taken a survey about (insert topic here) in the past 3 months, always choose No for the same reason as above.
I've also noticed that some surveys seem easier to qualify for than others, for example the Opinion Surveys are usually very good, but only allow one completed survey a day. Because of these surveys and the usual videos, I have managed to sign two scouts and buy a 50 Rest Pack for 146T so far this season. I started on 650T. I now have 707T.