Ex agreed to pay direct but is now missing payments

Been split from my ex partner 4 years and he agreed to pay £80 a month for 2 children only 1 year ago he has missed 2 payments and i dont know what else to do gave him chance after chance just calculated what i should be getting and im gobamacked am i dealing with this in the right way as i dont know what else to do i dont know his actual income i did a quess at 1000 a month i never asked for much for the kids but im sick of been made a fool of.