Usually, this means your preparation (everything before your take back) is not as good as it should be, but I'll go through 3 typical mistakes which might cause you some troubles. Tennis is a game where you have to give yourself the space, time and solid posture to hit a good shot, but amateurs often privilege the lazy option and, consequently, they hit far worst shots than they could.
You probably play "just in time" tennis. When the game slows down, you also slow yourself down: instead of rushing to the area surrounding the ball to take it as early and as aggressively as possible, even giving yourself some time to make very fine adjustments, you wait... and end up playing it in a less than ideal position. Is that what happens? I would guess so. If your strokes are fine, but start to feel soft and inconsistent once the game slows down, that's likely your problem: you don't prepare early and properly.
Another thing might be your posture: some players bend at their waist, sideways or forward, to hit low balls. It's better to keep an athletic position and bend your knees, getting your whole upper body lower instead of only trying to lower your arm. Look at Federer here:
That's the sort of thing you WANT to do: you get your whole body down.
The third tip I can give you also regards your preparation. Some players think that preparing early means waiting with the racket already taken back, but it's a mistake. If you want to wait for a tennis ball, you wait for it sideways, both hands on the racket. Again, a picture of Federer doing this properly:
In that position, you can wait without this pause to affect the quality of your stroke, but some people wait in this second position, which DOES inhibit their ability to strike hard and consistently.
In this picture, Federer does what's the right thing: you begin your swing (take back included) only once you're ready to hit the ball. What I wanted to show wasn't so much the right thing to do as the wrong one and Federer's posture here resembles what you see amateurs doing when they wait for a ball... Well, he might still be in too good a position, but you get the point. If you have to wait, mimic the first picture.