Problems with Expression Blend and Custom controls (including Telerik)

Has anybody else run into problems using Expression Blend to modify pages that include custom controls (and if so, have you found any solutions). Sometimes, things are fine. But then, especially, it seems, once a project grows in size and/or complexity, that Blend doesn't recognize custom properties (in other words if I have a xaml control called phoneTextBox, the width and margins will show fine in intellisense and not cause any errors, but any property that I added myself will NOT be seen, will cause an error, and as a result, will not let me view the page.

This is also happening with third party controls (specifically Telerik).

Has anybody any insights?
Capture.PNG