What Cause Elevated Blood Pressure Despite Taking Ziac?

I AM TAKING ZIAC FOR HIGH BLOOD PRESSURE. 150MG. IT FOR THE MOST PART WORKS BUT MY BLOOD PRESSURE STILL TENDS TO BE HIGH SOMETIMES ESPECIALLY TOP NUMBER SOMETIMES IN THE 150-160 RANGE. I AM 55 MALE ACTIVE WORKOUT AND WATCH MY DIET. I WEIGH 180 POUNDS. ANY SUGGESTIONS.