When one is working with 1,1 miliohm mosfets, the usual parasitic resistances that one usually disregards (i.e. solder joints, PWB traces, connectors) are no longer negligible.
I would measure the voltage drop right at the Mosfet terminals, to determine whether the loss is there or somewhere else.
Also, ensure that the Mosfet is fully enhanced. I've not read the datasheet, but please read it and determine what Vgs is required to achieve the specified RDSon. Then as FvM suggests, measure Vgs to see if you are meeting that requirement.