My dad is on medication for high blood pressure, but his blood pressure is still high...why?

My dad is on three pills to lower his blood pressure, and takes 2 shots of apple cider vinegar a day. He does not eat horribly and gets outside frequently (although he could afford to go on a few more walks per week) His last blood pressure pill our Dr. put him on about a month ago because his blood pressure was so high and it was supposed to stabilize it and bring it down, but it didn't. His blood pressure is still much to high... 140 over 100 somedays, then others 150 over 90....etc. either way the bottom number is supposed to be at 80. It has only been 80 about three times. Any suggestions or answers as to why it is still so high?