SYS21 - AR Trend Analysis

I have a strange one with the calculation of Trend Analysis (Debtor Days,
Days sales outstanding, average days to pay etc).

The Avg Days to Pay and SDP seem to be calculating to zero, while DD and DSO
have values?

Each period has transactions (invoices, credits and payments), but has no
impact on what trend/performance is calculated.

To make the situation even more confusing the manuals explanation for these
figures is that the client is over 12 months behind in rolling his AR (and
yes, CS and GL).  So we are rolling forward period by period running the
Stats Update in-between each period end.

Any idea why the calculations do not seem to function?

Regards

Michael