Check whether any of the strings have frayed. A frayed string will be close to breaking. A loss in string tension is another reason to consider restringing. A loss in tension will reduce the amount of control and power you are able to generate which will affect the performance of the racket.
Signs your strings have lost tension: your racket is beginning to feel 'dead' or lifeless, you are having to swing harder to get the ball to go deeper, you make good contact with the ball but aren’t getting as many solid contacts and the sound has changed when making contact with the ball.