This will depend on how those maintenance payments were agreed. If there is a court order setting out the terms, then you should take legal advice immediately to find out if you can enforce those terms by going back to the court.
If the payments were agreed amicably originally or through an assessment by the Child Maintenance Service (CMS), you need to use the CMS to help you, and we would advise you to contact them urgently. If the CMS does not seem to be dealing with your case promptly, or fairly, then you may need further legal advice to assist.