"Usually". a mare will come into foal heat about 10 days after foaling. If they come in before the 10 days and are bred, they "usually" will not take. If they come in after 10 days, you have a better chance of getting them in foal during foal heat.
Often, they will not show signs of foal heat (but you can tell they are in foal heat by the diarrhea on the foals butt) But, when you try to breed the mare to the stallion, she wants no part of it.
Then it can be a couple of months before they come into heat again. The nursing foal often keeps the mare from coming in. I have several who I have been trying every 3 days who have foals that are 2 months old, and still no heat cycle.
It can be very frustrating. Sometimes you have to wait until the foal is weaned to get the mare to come into heat.