Okay, the issue here is you have specified a default namespace (missed that)
Try the following code:
XDocument doc = XDocument.Parse(response);
XNamespace ns = "http://services.sample.com/forms";
IEnumerable<xelement> descendants = doc.Descendants(ns + "form");
foreach (XElement d in descendants)
{
}
</xelement>
In your example XML the last
forms
tag is missing, I assume that's not the case with the actual response?