Do you have a stallion on the place? If not, she may not show. Some mares will not show to geldings or other mares. I have one who will not show to MY stallion, period. She just had a foal by him, but we had to ultrasound her to see when she was ready to breed in order to accomplish it!
Both our mares have silent heats that even breeding stallions don't pick up on. The only way I know they are cycling is to start checking daily in early spring, mark on the barn calendar, and check again in 28 days, which is their normal cycle length. Fortunately both mares tend to be in heat within 24 hours of each other.
You may have a mare who doesn't show, it's sure nice to not have one marish, but that makes it hard if you want to breed her! :p