My ex-husband did not pay alimony for one month.

He has paid since then, but how do I get the back support?

You would need to go back to court and show the judge that the alimony was unpaid. Additionally, you would ask for interest (the current legal rate is 10% per annum) to be applied towards the balance, called arrears.

Keeps on paying every end of the month court order every 15th now missed a payment

Can you please clarify your question?