[Statistica - Nested Anova] How to do a two-way nested anova?


I'm trying to do a two-way nested anova in Statistica, but the results do not seen to be a two way model. In my study I need to compare if the abundances (of taxa) differ between my 5 sampling units (sites) considering 2 periods (spring and auttumn).
So, I have 2 randon factors (site and periods) and 3 replications per site (my sampling unit).
But, when I perform the two way nested anova (with replications nested with sites), the results don't show the interaction between the 2 factors (site x period).

Is there anything wrong? I chose in Statistica:
...Nested anova----> dependent factor (abundances)---->categorical factors (period, replications and sampling unit)....
---->Between effects---->replications nested in sites
The results were the p-values of period, site and raplications. But not period x site... =/

Thanks in advance!