Shesinthebarn - I see that you are located in Canada. How is the weather where you are? Mares up in the northern regions have a shut down winter period to their cycles in which they will build follicles then recede/shrink them without ovulating. I'm in Washington and while most of my mares are cycling normally, there are a few who aren't quite there yet. They need one more cycle to pop out of it. Or, we give them a shot of Lutalyse which should bounce them into a normal cycle.
Does your mare have any edema? What does your vet say? (Or whoever is doing the ultrasounding. I do my own.) If a mare is building then receding follicles without the presence of edema in the lining of the uterus (very visible on ultrasound....pictures below.) ....then it's a sure bet that your mare is in transitional heat. After a few transitional cycles and some good weather (or a shot from the vet) your mare will come into normal, breed-able heat. Good luck! 01-59-16.jpg
The black, splotchy fluid is edema. Mares in normal heat will have edema. (though the amount can vary) Edema in the horn.jpg
Edema in the horn. The wagonwheel/sliced orange effect. Mistichall 14 day - Copy.jpg
Mare who is NOT in heat and is 14 days pregnant. (the black circle) I outlined the uterus in white. See how there is no edema? No splotchy fluid? A mare who is not in heat will look like this, even when they aren't pregnant. Transitional mares will not have edema. That doesn't necessary mean you can't breed them without edema or on a transitional cycle, but you will probably need the drug HCG to force ovulation after breeding (on a follicle of the correct size, 35mm or larger) in transitional mares.