Error in sending PIDX to vendor

we are trying send PIDX xml to a vendor…with verisigned Certificate…From last year to till june of this year,everything is going smooth…and we didnt changed any of our certificates this july and we are continuing with the same one till now, but now a days we are not getting acknowledgement as “completed” instead we are getting as “waiting”…so i developed a service which picks the documents whose userstatus is “WAITING” …and reprocessed them all …now after reprocessing it gives acknowledgement “complete”…I am not sure why exactly in first scenario its not going thru and second time if we reprocess it, its going successfull…i asked manager to chheck the traffic of transaction, or log the contentID,delevery header when we are reprocessing so that we can capture the communication…if at all if we have the issues with the code or certificates it shouldnt work at earlier and after reprocessing too…but now it is not working initially and if we reprocess it it works fine…

the is as follows…Outbound Flatfile to Canonical to PIDX to PIDX envelope to vendor…
And we sending it to vendor as PIDX envelope using Rosattenet …
is there any other solution>?

Wm_Susheel
JPMC