Missing daily reports for days with heavy traffic

[Labsy]

Hi,

I receive (ZCS Default) summary report on a daily basis, except on days when traffic graphs show extreme traffic.
For example, average traffic is around 20.000 msgs/day, and when for unknown reason once per 2 months it raises up to 500.000 per day, well, forr this day I receive no summary report?!

I do not monitor traffic on daily basis, so I found 2 such spikes in past 6 months. It is too far away to search the logs.
But I have CPU and DISK graphs, and none of them showed some extreme utilisation.

What could be the cause for missing reports?
CPU too busy? I doubt that, no indication of such.
Any idea?