I had the same issue. He is what I found:
1) make sure all projects are using the same Framework (this is crucial!)
2) in Tools/Options>Debugging>General make sure "Enable Just My Code (Managed Only) is NOT ticked
3) in Tools/Options>Debugging>Symbols clear any cached symbols, untick and delete all folder locations under the "Symbols file (.pdb) locations" listbox except the default "Microsoft Symbol Servers" but still untick it too. Also delete any static paths in the "Cache symbols in this directory" textbox. Click the "Empty Symbols Cache" button. Finally make sure the "Only specified modules" radio button is ticked.
4) in the Build/Configuration Manager menu for all projects make sure the configuration is in Debug mode.