its not clear from your post if the BD14 cam is new or used.

However, if the head has been overheated at some point it might have warped and when that happens the cam journals wear down (not the ally caps for some reason).

it may be that an old (worn) cam still turns fine in the head but a new one which is the correct and slightly bigger diameter won't.

the cams have a machined circle on them which is not used by the engine but is a reference to the 4 journal diameters when the cam is new.

measure this with a vernier/micrometer then measure the journals to check that they are not worn on one of the cams, this might give you some idea of what the problem is.

another check for a warped head (or one that was once warped and caused cam wear but the warp was apparently fixed by facing off the head afterwards) is to lie a dead straight edge across the lower bearings and see if it rocks (when they warp the middle 2 rise up slightly)