What aggregation function are you using?.
May it be that the maximum value of OrderId does not have any information for Season?.
If that were the case, a count statement, for instance would provide information:
But a Sum() statement would return null result until you select one customer:
May that be the case?.
Hope it helps.
the expression doesn't work because
when you use set analysis it like making a selection in the document so the max(orderid) in the set analysis
will give back the max orderid in the whole document and not to each customer
you need to use some sort of if function so it will be calculated for each customer ,
will try to create an example later.