Adwords API; retrieving keywords for an account

Question

I am building a tool that needs to retrieve all of an Adwords user's keywords. When I test this tool, it performs as expected about 1/2 the time, retrieving all of a user's keywords. For the other half, when I send my SOAP request to the Adwords API, I receive a valid response from the API (I.E. no error is reported) with 0 keywords. These users assure me that they do indeed have keywords in their account.

Does anyone here have any experience with this? Below is my request, and an empty result. This has been challenging for me to debug, as I don't have access to the user accounts who are reporting no results. All the accounts I have access to perform as expected.

I am using the V200909 version of the API, as that was the latest when I started this project. I am open to upgrading to one of the newer versions, is it possible this would help? Thank you so much for your time, I am really stuck!

Request

<SOAP-ENV:Envelope 
    xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" 
    xmlns:ns1="https://adwords.google.com/api/adwords/cm/v200909" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"&gt;
    <SOAP-ENV:Header>
        <ns1:RequestHeader xsi:type="ns1:RequestHeader">        
        ...authtoken, dev-token, app-token,user-agent, 
        </ns1:RequestHeader>
    </SOAP-ENV:Header>
    <SOAP-ENV:Body>
        <ns1:get xsi:type="get">
          <ns1:selector xsi:type="ns1:AdGroupCriterionSelector"></ns1:selector>
        </ns1:get>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Response

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"&gt;
    ...header
    <soap:Body>
        <getResponse xmlns="https://adwords.google.com/api/adwords/cm/v200909"&gt;
        <rval>
            <totalNumEntries>0</totalNumEntries>
        <Page.Type>AdGroupCriterionPage</Page.Type>
        </rval>
    </getResponse>
    </soap:Body>
</soap:Envelope>