Payments not issued to you – This may be due to your ex’s employer not correctly sending the payments to the agency, in which case the funds may still need to be allocated.
Escalate this to a written complaint, sent signed for/keep copies (there’s a list of contact addresses at http://www.afairercsaforall.co.uk), also send a copy to your local MP, requesting their intervention.

As your ex has declared the information to the agency, he will receive a reduction of 25% now 3 (assuming they are all relevant) children are in his household. If your ex and his partner are in receipt of WTC (depending on who is the higher earner) and CTC, these figures can be included when maintenance is calculated. Request a re assessment in writing/via email, keep copies etc.