The first thing you should do is to take legal advice on the issue. It may be that there is a legitimate reason why your ex-partner cannot continue to pay maintenance at the previous rate, such as becoming unemployed or having a reduced income. If there has been a change of circumstances the onus is on your ex-partner to make an application to vary the maintenance downwards. They should not reduce the child maintenance without your agreement or an order of the court.
If there are maintenance arrears you are entitled to begin enforcement proceedings. Thought must be given before issuing enforcement proceedings, as if there is a legitimate reason for reducing the maintenance, it is likely that the enforcement proceedings will be put on hold to allow for the issue of maintenance to be dealt with substantively.