I have recently purchased a used (but remarkable condition) atx 400mm f5.6 with an ef mount. This works on my 6D, allowing for aperture and focus control from the body. 

However, my 70D doesnt register it at all: display shows aperture of 0.0 and selects manual focus in drive menu.  While the lens is functional, it would be nice for it to work with the higher frame rate of the 70D. I also have the atx 300mm f4 which works perfectly on both (hence this purchase).

What should I check / examine / investigate to get this working on my 70D?
Has anyone experienced this or similar (old lens -> new body)?