Hey there;
First question is how old is your female?
The next comment is it sounds like she has what are called "silent heats" ... where it's really hard to tell that they're in heat at all (no swelling, don't bleed much, keep themselves clean, etc.). The two best ways to monitor bitches with silent heat cycles are:
1) Keep her in an above-ground pen, where she's not getting dirty, and where you can see what little blod spots are there
on the wood when she does come in; or
2) If she can be an indoor dog, keep her inside with a friendly male ... and, trust me, when she comes in heat
his nose will let you know when she's ready ... because he'll suddenly start licking her and trying to mount her all the time
Good luck,
Jack