Assuming the agreement became binding then it becomes an open and binding contract between the parties. It is rare for the non-payment to be deliberate but it is often an oversight.
You should chase the payment with your ex-employer direct or your solicitor who provided advice on the agreement could contact them on your behalf.
Normally this is then resolved very swiftly. The issue sometimes arises if you have not complied with your side of the contract such as return of company property or that you are in breach of a warranty or other terms of the agreement. If so you will need to take further advice. A failure to pay could amount to a breach of contract.