Most likely this is the reason.
You had a survey that you had previously completed reversed. They did not accept your completed survey and as a result Clixsense were not able to credit you for it.
Reversals usually happen after their manual verification deems an offer or survey invalid for quality reasons. They may also disqualify you for security reasons so make sure that you take your time to read each question and time to respond to them. If you RUSH through a survey or offer or if you fail QUALITY questions or if you are caught as a STRAIGHT LINER you will not be compensated for the survey (even if you might see the congratulations screen) and your answers will be invalidated.
Phoenix had the same problem yesterday.
You could contact support and ask, but in reality, Crowdflower - research company makes the decisions not Clixsense.