Card boots on xM Rev C but not Rev A2

I have a several SD cards that boot fine on my xM Rev C board but not on my xM Rev A2 board. These include cards I’ve built with narcissus, and also the xM test beta 4-25 card supplied with the xM Rev C. These all ‘hang’ (although the sd card light keeps flashing) on the Rev A2 immediately after

Uncompressing Linux… … done, booting the kernel.

However I believe the board is OK as I have other cards (incidentally Ubuntu) running fine. I’ve drawn a blank locating information on this from search’s: the closes I could see was Where its essentially Problem #2, but these seam to focus on issues with bootargs being passed which do not tie with the Kernel being loaded, hence are not applicable here.

Any advice on where to look much appreciated.