I am sorry to hear your dilemma. We need ALOT of information in order to correctly answer your question as to why they are not laying. It may help if you let us know:
What breeds?
Coop size in relation to how many you have.
Free range or all cooped up?
Location, size and how high up are your nesting boxes?
A picture of the inside of your coop and/or run?
A description, good one, of of the inside of your coop: type of roost, type and how many boxes, how much light is getting inside the coop.
NO, NO and NO to the question that mixed breed flocks don't lay like full breed flocks. This is just silly. I have over 10 years of having mixed breed flocks with no laying problems such as not laying due to mixed breeds. You can use this site as proof that this is a silly suggestion. Many of us have mixed breed flocks!
Yes, 18-25 weeks is about the time that most heritage and rare breeds start laying but keep in mind; it is about winter time and normally most flocks start slowing down on production. For example: I have 15 hens and was getting about 10-15 eggs per day and now I am getting around 2-4 a day just in the last two weeks because of the light and environment factors that happen during fall. I expect this every year!
Also keep in mind that new chickens go thru atleast 3 light and one full molting periods before they start to lay. You may just have the same problem as most people do when they buy their chicks mid-year, they grow out and feather out then go right into molt before winter hence no eggs until spring depending on the breed. Yes it's frustrating but that may be the case. You'll find this true even if you were to go out right now and buy some hens that are laying.