It is a fact of nature that the strong will take advantage of the weak.

The possible solution depends on possibility many variables, such as.

Are you as a subcontractor easy to be replaced?  If so, look to replace the account and let it become someone else’s problem. 

Are you a critical supplier?   If so, now you possibly have “leverage” .

If the account is more than 10% of your business and the late payments are affecting cash flow, you may want to offer a discount for early or prompt payment. ie. 2% ten days.  At the same time try to increase other accounts or bring in new accounts to decrease reliance on the slow paying account.

Thisis just a few thoughts to get your mind in the correct place.