ASP.NET custom control: Element is not a known element. This can occur if there is a compilation error in the Web site

Hi Experts,
I'm developing a custom control called MyDateTimeControl. When I register the control to an aspx file in VS2005 like this:
        <%@ Register TagPrefix="UC" TagName="MyDateTimeControl" Src="~/UserControls/MyDateTimeControl.ascx" %>
The tag "~/UserControls/MyDateTimeControl.ascx"  is underlined in red and the IDE says that "File '~/UserControls/MyDateTimeControl.ascx' was not found".

In addition, when I declare an instance of the control:
        <UC:MyDateTimeControl runat="server" id="dtcFrom" ></UC:MyDateTimeControl >
I get this error: "Element MyDateTimeControl is not a known element. This can occur if there is a compilation error in the Web site"

However, I can compile the project and it runs very well. The problem is I cannot use intellisence and the designer.cs file is not regenerated automatically whenever a new control is added anymore.

Could any one show me how to eliminate this annoying problem?
Thank you in advance.