Sounds like the splines inside the top of the plastic agitator are stripped out. When you turn the bolt, you are spinning the agitator shaft inside the top of the plastic agitator. You will have to brake off the old agitator and replace it with a new one. The last one I did was able to purchase just the bottom half of the agitator and use the old top half with the spiral part. It might not split with a chisel. If not try cutting it off with a Dremmel. Good Luck!