It happens - just luck or (bad) luck of the draw.
Over the past 2 days I've successfully completed 5 surveys for credits -- had 1 or 2 get pretty far only to tell me I'm no good.
WGT doesn't really have any control once you click on that link to go to an external site to take the survey I think.
They are in control about whether they payout the credits or not, not you and not WGT. Some of them for sure monitor your responses and can detect if you're being genuine or not. Not saying you're doing that, but just be aware of it.