Make sure you still have line of site from the dish (no buildings or trees in the way blocking the signal). You can easily see where the dish is pointing.
Try cleaning the actual dish. A good Sky signal requires it to "bounce" off the dish into the LNB. If the dish is dirty the signal may not be of high quality.
You can test your signal strength, press the Services button on your remote control and find the place where it tells you your signal strngth and signal quality.