This will all be extremely identifying but I don't care.
Ex and I have an 11 year old DD together. We were never married and split up when she was 18 months old. When I first claimed Child Maintenance through the CSA he didn't pay so ended up with a DEO (after many phonecalls and chasing the CSA from me).
After a few years he changed his job and the CSA took a year recalculation his payments. In that time I didn't receive a penny. I begged and pleaded with ex to call them to make some small contributions as any amount would have helped but he refused.
When the CSA final sent him a schedule of payments he didn't pay anything. The CSA said they had to give him a chance to pay before setting up a DEO. Three months later they finally set up a DEO and as they were recouping the arrears from the year ex hadn't paid a penny the payments taken out were double what they should have been.
Suddenly ex wanted to make an arrangement between us and for him to pay me direct
... I told him where to go. The DEO has been in place happily for a few years until now.
I received a letter last year saying that our case was being moved to the CMS system meaning he could pay me direct, I could ask for a reassessment of what he pays for a one-off fee of £20 but he still pays me direct or I could ask for the reassessment and collection service which would be a monthly fee of 4% from the money collected from him and an additional 20% collected from him in addition to the maintenance.
We discussed it and due to the additional fees gave him my bank details so he could pay me direct. Well this month was the first month it was supposed to happen but surprise surprise no money. WIBU to just ask for the collection service without further discussion with him?
Other background is he's a useless dad. We live about 40 miles from each other DD will be lucky if she sees him twice a year. We've only just moved and previously lived a mile away from him but she still would only see him twice a year if lucky.
I've always had the door open and used to invite him to school events and send him reports etc but he was always busy or too much effort etc. Last year I tried to invite him to her leaver's service to find his number didn't work
. I only got a contact number for him when he called me six months later 
He has now changed his number again as I tried to call him but was out of order. I have a new number for him which I got from his 16 year old DD who lives with him (via facebook) and have texted him but I feel like saying "fuck it" and just going straight to CMS and say tough if it costs him more.
We're not talking masses here; it's £100 a month but I work full time earning pittance. I have not lived with anyone since we split up so it's been just me and DD.
ARRRRGGGGHHHH