Session variables set to Nothing after Webservice call (or during?, or when?)

I have a page with a DynamicPopulateExtender javascript call to a Webservice. The javascript is triggered by a TextBox onchange event.
The event fires and the service itself works nicely but a side-effect is causing problems:
When a Button is pressed, the Button_Click event fires.  In that subroutine I check for certain Session variables. To my astonishement, ALL Session variables have been set to Nothing!!
The service is defined with this line:
<WebMethod(EnableSession:=True)>
Yet, when I debug inside the Webservice, ALL Session variables have been set to Nothing!!
I looked around the forums for something similar but could not find.  I would really appreciate help.
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