I have one like this; but I've had no trouble getting her pregnant. For a variety of reasons I synchronize all my mares (I only have 2-3 to breed each year) which may be one reason why I haven't had trouble - I just count, use the calendar and palpate without bothering to see if she's in heat.
Not all mares show overt heat behavior --particularly if there is no stallion present --even though they are cycling just fine. Even at the repro clinic, I've had mares that were in the process of ovulating and didn't show much behavioral response to being teased by the teaser stallion. Palpation and/or ultrasounds will give reliable information.