I haven't used a Beseler 23c so don't have first-hand answer for you. But looks like a flat lens board would be appropriate, part number 8021

If that's the board you are using, maybe you need to raise or lower the entire enlarger head and try focusing again. It's not that you set the focus once and then zoom without having to focus again... You have to work both the height and the focus until you get the size enlargement you want and it is in focus.