Sorry, I missed that you said this was FP4.

FP4 normally stains very intensely. Plus-X stains a little moreso but also has more general stain. I find the stain on both to be very obvious.

If you're tube or rotary developing your sheet film the presoak might be more necessary - you can always experiment. For now I'm tray developing with PMK, or developing 35mm and 120 in standard tanks.

Klainmeister has it right - I'm not sure if you're mixing up your working solution well in advance. You should mix it up immediately before use. I start using it within a minute of mixing it - there's no need to wait. Pour out your water, measure out your A and B solutions (I pour the A into the water as soon as I measure it), and dump the B solution into the A + water as soon as you're ready to develop.