There are several possible explanations. Notice the two other threads here concerning how heat causes the reduction in bloom size. If you feel increased heat isn't the issue, it may easily be that the soil it has grown in has finally depleted, reducing the available oxygen available to the roots, and perhaps the nutrients. Drainage suffers as soil breaks down. The plant could need repotting to correct the drainage, improve the water holding capabilities of the soil and improve the nutrition available to the plant from the soil. I'm sure there may be other possibilities, but these come to mind first. Kim