brush and pencil strokes won't flatten

I can’t flatten either brush or pencil strokes. I tried all ‘‘alt+shift+f’’, ‘‘optimize panel’’,’‘tool properties’’. Hope somebody can help me.

Which version and tier level are you using?

How are you testing to see whether it is not working?

Can you describe each step?

Curious, have you also tried using Auto-flatten to see whether you can produce anything flattened?