Holding serve in the modern game of tennis has become more and more difficult. Players can now return serve more often and even use the speed of the serve to add to the offensive nature of their return.
Holding serve now requires a one two punch on most points-a good serve and a good first shot. Even then, some players (think Nadal, Djokovic, Kerber, etc.) have become so adept at playing defense and turning that defense into offense, that it can take even more than two very good shots to win most of the points on your serve.
It has also become much more difficult to back up good serving by moving forward and using volleys to end the point. Again, the master defenders are great at passing the net player, making most of them indecisive and not able to cover the net as well as they have in the past.
My belief is that there will be a new wave of players who are capable of using their forward movement more, to help players hold serve more often and in crucial stages of the match.
Practice the transition game and your volleying technique as well as serving and first shot, to improve the number of games that you hold serve in your matches.