I asked once:
"I received an email dated 22 July (2016) advising there was a new survey available . Yet after logging into my account today (26 July) the My Survey page under the heading "Surveys for you" states: "There are no surveys currently, please check back later."
and got this response:
"For some of our surveys, we are only looking for a small number of people to participate. In this instance, we reached our target number of responses very quickly thanks to a very strong response from our members. Unfortunately, by the time you attempted to do the survey it had reached its goal and was closed.
We understand this can be frustrating and we do try to make as many surveys as possible available to a large number of our members, but that is not always possible. As a result, we encourage members to take part in surveys as soon as possible."