First of all dogs should not even be bred till two years, the hips n elbows should be OFA certified A normal or A Fast Normal.
If they were in heat in Jan and one had a litter, it must of been only little over a year old?? And, some females only cycle once a year, the mith of dogs cycling every six months is just not so nor is a 21 day heat cycle so.
I have females the swell first week of heats, drips a few days, ready to stand and it might only be the tenth day of total heat days, if u do not know your girls they can slip out quick as they come in and u missed her.
I have been breeding dogs for many many years, I am probably the oldest member her at 67 years. I bred, trained and showed horses for some 35 years before really going into serious dog breeding. I do believe in keeping with the standard with my West German Shepherds, to keep the size normal, to keep the weight normal and have excellent lines and a stud to cover faults the female has to use on her. Do NOT rely on days and months, for females can cycle way off and not at all for long periods. I know I post up a female is to be in heat so n so month, she goes on for another 8 months ........no heat. One cannot predict a heat nor when nor how long.
This is where AI comes in handy, but our small town my vets want no parts of handling nor storing or responsibility of. So, I am left to keep a sharp eye a dog calendar, and a constant check on each girl and I have 5 of them. Use your own judegment, do not rely on others as breeding is very riskea, stressful and dangerous as is whelping puppies.