This is what is known as a "silent heat."

A silent heat is one where a bitch is reproductively fertile, however during her heat cycles she gives no indication (or little indication) that she's heat.

My Tuffy bitch was like this. I overcame it by keeping her in an above-ground pen. What little blood Tuffy discharged was almost impossible to see because she was very fastidious about keeping herself clean.

The very little bit of blood she would discharge in her heat cycle, that would drip out, was easily-seen on the wood of the pen, whereas on the ground (in the dirt) you couldn't see anything.

An experienced breeder can also tell by the way of bitch acts whether she is "ready" or not. However, the best way to tell is to get an Ovulation Test Kit.

Hope this helps,

Jack