Certain developers like D-76 and Rodinal (with other analogs like R09) do not produce full rated speed. So its is not surprising that Fomapan 400 only yields an EI of 250 in Rodinal. Manufacturers design their films and developers together. My advice would then be to look into one of Foma's recommended developers like Hydrofen. I published the formula and developing times on APUG some time ago.
I hate to be a stickler, but how do you know know D-76 does not produce "full rated speed"?