My ex has moved jobs several times to avoid paying money he owes towards the upkeep of his children. He, like so many other men mentioned here, seems to think that he owes us nothing because I am Mrs Evil-who-must-be-punished ( 7 years ago I ended a pretty abusive marriage after 18 years and 3 children) , and he is convinced the kids will understand and accept his point of view: the eldest is now 20, and thinks his dad's attitude stinks, by the way.
I deal with his refusal to pay maintenance by phoning the CSA relentlessly every time he goes back to work, and insisting on having payments made to me through an attachment to his earnings, ie, payments are made to me directly from his employer, and my ex has no say in the matter. I always have to insist that this is done, and the CSA is often reluctant, but I ask and ask, phone every other day, ask to speak to supervisors if I am not getting any joy, etc. A supervisor/manager at the CSA told me that this was the best way of forcing them, the CSA, to deal with my case quickly as they are always busy, and the most insistent people will be dealt with first.
I've found that the CSA works very slowly but that they have to respond to requests for action and information, so the more frequently you phone, the more quickly they have to deal with your case.
My ex has just started working again after a 7 month break, and I have started my pursuit of maintenance. He owes about £650 from last year. I expect to start getting some funds in about 12 weeks, if I insist on an attachment to earnings, and the payments made to me will initially include the arrears.
So, be firm, and be insistent, be very persistent, phone the CSA every couple of days, don't be fobbed off - these ex's need to contribute to their children's upbringing!!!
Oh, and never depend on the money the ex's send - I've learnt that the hard way.