One of the key reasons we use a tripod is because we need to shoot with a long exposure which we cannot hand hold. But sometimes we still can't seem to get a sharp photo. Here are a few things that might affect the sharpness of your photos.
The first consideration is the head and the tripod itself. Do you have a sturdy tripod with a sturdy head? If your tripod cost less than $50 new, it's probably not enough. You also need to make sure that the head is rated for the amount of weight you put on it. In other words, if you have a head rated for 1.5 lbs and your body and lens weigh 4 pounds, you will probably experience some slippage; usually about the time you decide to take the photo.
Is the camera firmly mounted to the tripod? Very often photographers do not tighten their camera enough to the tripod head or quick release plate. Make sure your camera is firmly mounted.
Another reason may be your image stabilization. One of the best technological advances in photography is the advent of image stabilization (aka vibration reduction). This technology allows us to take hand held pictures at slower shutter speeds without blur from our own hand movement. However, if you have your camera mounted to a tripod, you MUST turn off your stabilization system. If the camera does not detect motion, it will create motion.