If you watch your cows very close you will see mucos drain out of them 48 hours or so before they are in standing heat. I have seen it numerous times when flushing a cow or waiting for one to come in heat. At first i was like how did i miss her when i was checking all the time. and sure enough they would still come in heat. Just be patient. I have learned that over the years. Just watch for her to start acting different by sniffing other cows, bellering, arching her back when behind other cows and about 6 hours later she will be standing.