Of my 3 Red Star hens, one started laying at 20 weeks, another started at about 21 weeks, and the third started at about 23 weeks.
Recall that the day length has an impact on egg laying. So, while some here may report that they got eggs from pullets that were 17 weeks or younger, that may have been at a time of year when the day length was pretty long.
Right now, we are on the other side of the shortest days, but still we are lucky to get about 11-12 hours of daylight. Fourteen hours is supposed to be the minimum amount of daylight for reliable egg-laying.
I'd say you'll just have to be patient. Certainly make sure they know *where* to lay by planting decoys in the nestbox, but realize that they may need more daylight to start laying.
BTW, my Delaware pullet started laying at 23 weeks, and gave her first egg the week before the winter solstice. I was quite impressed that she did so considering the short day length. Delawares are supposed to be good winter layers per Henderson's chart, and I'm giving them a thumbs up myself! She gives me an egg about every other day. They are still rather small -- typical for a pullet -- but they taste just fine!