How do I troubleshoot debug symbols not loading in an asmx page?

I have a web page that calls a web service contained in an asmx page.  In the past, I have been able to step from the web page javascript right to the asmx page code.  But now when I try to debug, the asmx page's breakpoints show the symbols not loaded icon and of course debugging no longer works.  In another project, I changed the project type from project with a pbd file to just a web site and debugging resumted functioning.  This time, the project is already a web site with one asmx page.  Since the file has not compiled components, and the debbugging settings appear to be correct, I'm not sure what to check.  Web site searches have proven fruitless and what I need is someone who has done this before.  If you know what I should check and know what I'm talking about, please provide suggestions as to what to do.  Thanks all!