The problem with using nodes too close to a gateway is that the gateway receives the same packet on multiple frequencies (cross-talk). Chances are the response will be at the wrong frequency resulting in the node not receiving it.
Another possible cause of join errors is the transmissions by the gateway not being received by the node because of low signal strength. Just a (silly) question, are you sure your gateway antenna is connected to the right u.Fl connector? (Should be the one furthest away from the row of 20 pins)
What kind of antenna are you using for the gateway?