My initial thought is check for ABO compatibility, which is what exlimey is suggesting. How was the Jka antigen testing performed? Were controls run and did they work? Also, perform a DAT on the units. If they are DAT positive, your crossmatches are invalid. I doubt that would be the issue though. 4 donors all having positive DAT's? If you had a clear anti-Jka identified, and all other antigen groups ruled out homozygously, there is no reason the units should be XM incompatible if they are ABO matched.