Cannot single step into VCL source code anymore

Problem/Question/Abstract:

All of a sudden I cannot single step into the VCL source code anymore. The path to $(DELPHI)\Source\Vcl is in the Environment Options Directory and I also tried to add it to the Project Options > Debug Source Path but it did not help.
It appears that $(DELPHI)\ is correct since I added a absolute path to the list and D5 recognized this was the $(DELPHI)\ path and changed it back to $(DELPHI)\Source\Vcl automatically. What happened?

Answer:

Here are two possible reasons for your situation:

You may accidentally have unchecked "Use Debug DCUs" in Project | Options Compiler.

Perhaps you have switched "Build with run-time packages" on.