My experience is of a very bitter and resentful ex. I finished with him after years of an abusive, co-dependent relationship. No one has ever really said 'no' to him, and I did. He began immediately trying to punish me by disrupting child care (at that time he was SAHP, not through choice but through lots of factors including having lost his licence for drink driving).
I stood firm, rearranged all the care without him in the picture and he was outraged that I didn't beg him for help (I work FT). At that point he was seeing the dc but paying nothing as per (he was a cocklodger when with me, amongst other things).
Anyway, long story short, everything deteriorated to the point that he stopped seeing the dc 22 months ago. He blames me for this, of course.
It's all to do with resentment at me, as far as I can make out. He has failed to grasp that the dc are separate people from me. He is a fool.
As a result, I had to pursue him through the CMS. He lied to them about his income (cash in hand jobs etc). I still don't get why anyone would not want to support their dc, but hey ho.
Now, after a period on the dole, when they assessed him as having to pay me £6.72 a week for BOTH dc, he is apparently on the books with an employer. I will have to wait for his contribution to be reassessed upwards, plus he's in arrears so now they are going for an attachment of earnings.
In short, this man gives no f*s about his children, and is just desperate to 'get one over' on me. He must be very unhappy deep down.