Why are you not able to complete them? As far as I know YouGov doesn't disqualify you. And if you are not eligible for the survey, they instantly find a new one for you. Your time spent on their platform never goes waste. You are compensated for it. At least that's how, it works here for me.Here in Italy I receive more surveys to complete but practically none of them I manage to complete and therefore I don't earn anything. Yougov instead from what you tell me, here works the same way. In fact I noticed that the frequency with which it offers surveys is quite random, however like you I manage to reach the withdrawal quota about once a year.
Most of the people whom I know take close to a year to reach the payment threshold. It's a huge number of 50 dollars and takes time, even more if the survey invitations are not frequent.