CSA takes ex’s word even though I have proof to disprove her lies

I seem to be on the same boat as most dads. My ex said because maintenance was calculated low that I would pay school dinners etc which is around 60 a month plus school trips etc then I find out she been contacting csa asking her to get the payments from me and being backdated. When I spoke to csa they basically said if my ex doesn’t agree that this happened then it’s her word against mine , how stupid. I could get proof from school etc but it’s her word against mine .