Forecast Consumption in MRP only works in the first FC period

We are implementing MRP in our JDE XE environment (yes still on XE!) and want to use forecast consumption to avoid issues of mismatch between sales order and forecast timing.
I have set everything up and the forecast consumption is working - but only in the first forecast consumption period after the generation start date when I run R3482.
So if I generate the plan starting in May, all my May sales orders net off my forecast, but my June (and other future) sales orders do not.
If I generate the plan starting in June, the June sales orders net off the forecast, but not July onwards.
This is causing a situation where the Planned Work Orders are not being affected by the sales orders, only the forecast so I am potentially not making enough product.
I have FC entries in P3405 from Jan 2015 thru Dec 2016 and I have Shop Floor calendars and Fiscal Date pattern out to 2018. I am doing a full regen on each run and I have tried varying the generation periods between weeks and months but with no difference. The first FC period works correctly and the later ones do not.

Has anyone encountered this before? It seems to me I must have a setup issue, but cannot locate it.

Thanks, Tethys

Tethys - check you Item Branch records to make sure than not only is the Planning Fence Rule set to H but also the Planning Fence = 999
Forecast consumption will not work properly unless Planning Fence = 999
Regards - Vic