N-400: Failed to support your dependent

[baikal3]

You don't have evidence of making the payments, nor evidence of thorough efforts to make payments. So your N-400 will be denied if you apply soon.

I agree.

Raghucp1: until and unless you take care of the child support issue, your N-400 has little chance of being approved.
If you cannot get a response from your ex-wife, your best chance seems to be to work out some kind of an arrangement through the court that issued the original child support order.