It is difficult to detect estrus in lactating cows.  Ray Nebel at Virginia Tech University estimates that a lactating cow is in heat for less than 9 hours on average and only spends about 24 seconds or so being mounted.  Nebel also has shown that cows showing estrus poorly are just as fertile as the ones showing intense estrus - that means that when you miss estrus in a cow, you miss a good opportunity to get her pregnant.  

Estrus Detection Websites

Heat Detection and Timing of Insemination - from Select Sires