I don't breed dubia but I have a huge colony of discoids and assume care isn't much different. I started with 40 mixed nymphs so it was many months before I started seeing an increase in number. Here are some tricks that might help you in addition to patience.

1) Cover the sides of the tank if you haven't already or at least put it in a dark/private area. Roaches are shy and you won't get much breeding if they're always scurrying about trying to find better hiding places.

2) Keep fresh citrus in there. When I need to increase roach production I'll get a bag of oranges, cut them in half and just place the 2 halves(cut side facing up) in with my colony. For whatever reason citrus really seems to put roaches in the breeding mood. I'd just do 1 orange at a time and if it starts to mold before they finish it, trash what's left.