If the saw is not cutting straight, the first thing to check for is a dull blade. A dull blade will not only slow down the speed of cutting but will also prevent a straight cut. If the blade is obviously still sharp, it may not have been installed correctly. A band saw blade can accidentally be installed inside out.
If the blade is inside out, the teeth will point in the wrong direction and make a good cut impossible. The blade tension may also be too weak for the blades one is using. Usually, these problems can be fixed easily as there is nothing seriously wrong with the saw.