You can have holes in your service area IF you have roads that cannot be reached within the specified breaks. Unreachable lines can also be due to restrictions such as oneway. You can minimize/eliminate the holes by making sure that on the Polygon generation tab you choose the Generalized option and not the detailed option.
You also say that some positions are not correct. Where are these not correct? You can turn on the lines in the Line Generation and choose Split Lines at Breaks and then once the results are created, symbolize each line break value as well. This way you can verify that the lines and polygons match the break values.
Jay Sandhu