From what you have said, you and your American cousins definitely match, and you and your English cousins definitely match, but your English cousins don't match the American ones at all, in any shape or form, even at 5th to 8th cousin level.
This comes across as an illegitimacy at some stage, but where? For you to match with both sets, it would not seem to be in your family line.
The American matches start coming in a bit further down your list of matches, so I would think that is where the unknown comes in. The fact that the first American match shared significantly less DNA than your English matches, but supposedly from the same MRCA might indicate that your great grandmother had one child 'out of wedlock' or your great grandfather fathered an illegitimate child who was brought up in the same family as the rest. Thus, those descended from this branch, the American one, inherit a lot less DNA.
According to ISOGG, up to 10% of third cousins will not share enough DNA to show up as match. I know you are 2nd cousins, but if the American branch is descended from an illegitamacy, they will be further removed.
I think you'll just have to carry on digging, looking for any other possibilities as well. I'll be intetested to learn how this turns out.
Regards Margaret