What is body mechanism that triggers an elevated bp reading during office visits and normal at home? shouldnt medicine always be effective?

Sounds OK, no worry: BP naturally varies from hour to hour and even minute to minute, and can be very sensitive to even minor stresses or anxiety. Being on BP medications lowers the average BP over time, but does not prevent ups and downs due to innumerable factors like physical activity, stress, etc. Unless super high, a particular BP in the office doesn't matter. What's important is the everage over the day.