Many people have better success when playing against "hard" hitters than they do against "soft" hitters. The main reason is that you are just returning power that a hard hitter is giving you. When playing players that don't hit with power, you probably try to generate too much power, and this changes your swing and normal game.

You have to decide to play your game and your style without over hitting or becoming too aggressive. However, you don't want to try to "soften" up your strokes so much that you are playing your opponent's game.

In my opinion, footwork is a major factor when playing a softer hitter. When your opponent doesn't give you power, the ball doesn't arrive to you as quickly. Many people struggle because they don't move to the ball. They wait for the ball to come to them which it does more slowly. When this happens, players usually make a faster swing when good footwork would help solve the problem.

Another major factor is the mental game. How many times have you thought, "how can I lose to this guy? He doesn't even really hit the ball!" You have to stay focused, play your game, and remember that it's just a different style of play that you must make strategical and mental plans against. If you could hit "soft" shots and beat Sampras wouldn't you do it?

I would encourage you to practice against as many "soft" hitters as you can. Personally, as a collegiate player, I went through the same pains. I played great against a highly ranked serve and volley aggressive player and fell apart when playing the consistent baseliner that no one had heard of. You are not alone in your difficulties with this type of player, but you've probably learned to adjust against opponents with different types of spin, pace, and direction and you can also learn to overcome the "soft" hitter.