Why does my needle keep falling out?

I have a Singer Ultralock Serger, Model #14U-64A. I replaced the needles and now the right needle will not stay in. The left one stays in just like it is supposed to when the screw is tightened, but the right one falls out. I know I have them in the correct way with the flat part to the back. I have tried several times and it is always the same. Left one stays in, right one falls out. Help.

Thanks,