I had that with the Chambers with old style atacs and the platformed mx4(I think?).
Gave up on the shoes in the end, they were too flexy and could never get them to feel right regardless of pedal type...
I have mx4s on another bike and find them to be more supportive actually, just can't use the Chambers to ride my fixed as that's got the old style atacs.
If I ride in my Giro Chamber or Shimano DX shoes with my Atacs (old style plastic body) I feel like my feet are falling off the side of the pedals.
Don't get this feeling if I wear my Gearne G.Keiras,
My feet are rather bike, size 46/11.
I imagine it's something to do with the Chamber/DX shoes being less stiff and supportive.
Could it be cleat position?
It's making me consider going back to spd as I never had this issue riding either the Chambers or the DX shoes on m540 spuds.