Needed new blades so bought some and gave them the reg so got to be right. When I went to remove the old ones, I pulled the arm up and the blade dropped out, both sides. Then trying to install the new ones I have the same problem. The little plastic clip on the arm doesn't fit the blade. So I go back and make sure I've got the right ones and apparently they are. so am I missing a small part? Where do you lot get your blades from? Part number?