Hi Richy,
A NOT_FOUND response indicates that the click was never recorded by DoubleClick or by the DCM ad server (eg, it was collected from an AdWords click redirect but not through DoubleClick). Also, the timestamp of a conversion can be at most 28 days old from the time of submission. If this does not answer your question, could you please reply back with the request and response of the error in JSON format for a conversion that happened recently(in the last 7 days), so that we can troubleshoot further?