Ex husband child support probs!

Am I wrong for feeling done? So my ex husband and I share a son together and he only pays $250 for child support. If he has to be late I will sometimes try to work with him. I have bent over backwards to try to make things work for my son. I’ve tried giving him extra days and he cancels etc... well this is the 3rd month he is late on child support and I told him how I felt. I told him I feel as if my fiancé and I are doing everything. I’m not desperate for the money but don’t feel it’s not fair for him to just cancel and not pay and basically do nothing. I know some moms don’t have someone pay child support but my child doesn’t even stay with him he stays with his parents while he’s barely there. I showed up today to get my son and his gf was all shitty shaking her head and being hateful because they had to sell something I guess to get the money to pay for the child support. Is it just me or are they irresponsible? I get things happen but should save for it like it’s his son? I’m just getting tired of having to ask and him not paying it.. he’s always saying I have him strapped for money because of the $250! That’s not even what the court ordered him to pay I worked with him on the amount so that he could still live. Anyone else been through this its extremely frustrating!