Without actually seeing you bowl it is difficult to say what the exact cause is. It could be timing, it could be drift, it could be balance, it could be shoulder placement, etc. etc.
Basically for timing you just have to keep your mind inside the ball. Let the ball dictate the approach, and let the feet do what ever it takes to keep up.
The place where we put our focus, will dictate everything else. If we put our focus on the target, then the swing, the feet, everything else suffers, because the target doesn't care when we get there, or how. If we focus on the motion of the ball, everything else will follow the ball's motion, then just a split second before delivery shift focus to the target.
Watching the ball go all the way down the lane till it falls off the pindeck is critical. That ball is shouting information about what it is reading off the lane. We have to be prepared to receive the information and process it. It also tells us plenty from the time it contacts the first pin, until it falls off the back. That path through the pins and the deflection off the pins, are the encyclopedia of bowling.
It is common to bowl great games when your practicing. Nothing is at stake, no one is counting on you, there is no stress or tention in the muscles. But until you can replicate that feeling in league play, the scores will always be lower in league.
If you ever need to check your timing during play, just check where your sliding foot is, when the ball reaches the top of the backswing. At the moment the ball stops going up and seems to "float weightless" for a split second, you should be going into the slide. If your on time there, nothing else matters as far as timing goes. This is the proper leveraged position for delivery.