It could be any number of things. Including the one I take it you're looking for, namely some genetic inferiority on the part of the people in country A. But even if that were the entire cause it could still easily be the case that when someone moves from A to B their productivity (especially if expressed in monetary terms) increases dramatically.

I'm actually not quite sure what point you're arguing now. A few comments back, though, your claim was that Nancy was (nearly) contradicting herself by expecting immigrants to (1) be productive in their new country even though (2) their old country is the kind of place where it's really hard to be productive, on the grounds that for #2 to be true the people in the old country must be unproductive people.

It seems to me that for this argument to work you'd need counters to the following points (which have been made and which you haven't, as it seems to me, given any good counterargument to so far):

• There are lots of other ways in which the old country could make productivity harder than the new -- e.g., the ones I mention above.

  • Let me reiterate that these apply even if the old country's productivity is entirely a matter of permanent, unfixable genetic deficiencies in its people. Suppose the people of country A are substantially stupider and lazier than those of country B; this will lead to all kinds of structural problems in country A; but in country B it may well be that even someone substantially stupider and lazier than the average can still be productive. (Indeed I'm pretty sure many such people are.)
  • If the differences between A and B do indeed all arise in this way (which, incidentally, I think there are good reasons to think is far from the truth) then yes, if the scale of migration from A to B is large enough then it could make things worse rather than better overall. Given that the empirical evidence I'm aware of strongly suggests that migration to successful countries tends to make them better off, I think the onus is on you if you want to make the case that this actually happens at any credible level of migration.

• The people who move from country A to country B may be atypical of the people of country A, in ways that make them more likely overall to be productive in country B.

  • Your only response to this has been a handwavy dismissal, to the effect that that might have been true once but now immigration is too easy so it isn't any more. How about some evidence?