The difficulty arises when you don't have enough room to swing a hammer to tap it in squarely. You need to make up a little slide hammer which uses a rod and sliding weight to tap it in. Once the weight is guided it is much more effective. Depending on what you have in your workshop, try getting a socket that just fits inside the plug and fit it with a small extension. Next find a weight that just slides over the extension and strikes the socket evenly. Some mechanics apply a little grease to the plug on these hard to get ones to get them started a bit easier. Make sure the plug has not been distorted or the lip damaged in the previous attempts or it may not seal.