i've never had a female lay eggs without doing erm, business, lol, with a male first.
it is possible that although your snails are the right size according to your research to begin breeding, they may just not be ready. i'd say give them time, and if no eggs in say, the next few months, then you may possibly have all of one gender of snails. i've had briggs for about a year now and i still can't tell the difference between the gals and the guys, i just lucked out with getting one of each the first time around, lol.