Is your WebGrid and/or your WebCombo inside the WARP?

My assumption is that one control or the other is not in the WARP, and that you've passed the control's server-side ID to the WARP as part of the RefreshTargetIDs or TriggerControlIDs property.  This would work in most cases if you're not using master pages, since the control's server-side ID matches its client-side ID.  Once you put this on a master page or content page, the control is now in a naming container, and thus the client-side ID no longer matches the server-side ID.  The result is that WARP doesn't recognize your control as a control that it should be paying attention to.

If this assumption is correct, then the soltuion is to use the client-side ID of the control to the corresponding property instead.  This is most easily done by using methods on the WARP, such as AddRefreshTarget() and AddLinkedRequestTrigger(), and passing the control itself as a parameter.