Spray arm not functioning properly

Very simple. By hand, I unscrewed the old spray arm and screwed in the new one. One brief hangup. The old spray arm was very hard to unscrew. Rather than seeming to force it, I called the PartsSelect 800 number and got advice. The tech reassured me that all I had to to was unscrew it and put in new one. So I applied more force and old one came out. A breeze.