Null Results from successful syncMultipleLeads Calls? API Logs?

I've started seeing recently a couple scenarios where I was receiving a null-pointer on trying to get the results from the SuccessSyncMultipleLeads object on a call.  I wrapped this in a null check on the result of the call, and haven't seen a call where this isn't null, over the past week.

SuccessSyncMultipleLeads result = port.syncMultipleLeads(request, header);
if (result != null) {
// ...get the result.getResults() object and check for updates and inserts.

Is there a way to see logs of API calls?  I have a SmartLIst that confirms that the call updated thousands of records, so from that standpoint, the calls seem to be going through, but that result object comes back null and I can no longer report on the successful updates vs. inserts, as I had done previously.

Any thoughts on what might be happening here and how to troubleshoot are welcomed!

Thanks,
John