ASP.NET 2010 not see variable values sometimes

When I am debugging a C# 2010 web form application that is running against iis7 from my visual studio.net 2010 professional ide, I get messages like the following:

this Cannot obtain value of local or argument '' as it is not available at this instruction pointer, possibly because it has been optimized away. Damport.DImportDataContext fileOrServerOrConnection Cannot obtain value of local or argument 'fileOrServerOrConnection' as it is not available at this instruction pointer, possibly because it has been optimized away. string mapping Cannot obtain value of local or argument 'mapping' as it is not available at this instruction pointer, possibly because it has been optimized away. System.Data.Linq.Mapping.MappingSource

This kind of error message occurs in different sections of the code.  Some sections I do not have this problem.

Basically when this kind of error message occurs, I can not see the value in different variables. Thus can you tell me what I can do so that I can see the value of the variables when this error occurs?