Are you using XmlDocument.SelectNodes ?
If so, I suspect that you are running into a problem with the namespace on the third line -- which applies to all the elements within the GetPatientFullMedicationHistoryResponse element.
In order to use XPath to select the desired Element you need to specify a prefix to use by using an XmlNamespaceManager. (It's counter-intuitive.)
I don't have the code I use to do that handy and I still haven't gotten around to writing an article on it.
If I recall correctly, you'll need to use;
System.Xml.XmlNamespaceManager nsm = new System.Xml.XmlNamespaceManager ( Document.NameTable ) ;
nsm.AddNamespace ( someprefix , "https://secure.newcropaccounts.com/V7/webservices" ) ;
Then use someprefix as the prefix.