Just keep introducing them, 3 days on and 3 days off is good. Just because you don't see a lockup doesn't mean it isn't happening. I've got a male het albino that has produced several albinos for me over many years from a number of different girls and I have never seen him locked up. If you use a newspaper substrate, another good way to tell if you're getting actual breeding is to see reddish brown stripes on the paper the next day. That's from the female leaving tracks