In other cases, there may have been no scenarios that would have produced a metric in that group/interval. For example, if there were no interactions at all during your query interval, then there will be no data returned for answer time, talk time, etc. This is deliberate and the distinction between null/no-data and zero is an important one. In the case of timer data, a value of zero has logical significance--saying there was an answer time of zero would imply agents are picking up the phone instantly. The impact this would make on averages in aggregate would also be significant.
It's possibly a case that those metrics haven't been produced for that interval. Refining your query and/or increasing the interval time may be of help.