First, the configurations “Debug” and “Release” are just names, you could also name them “Jon” and “Carla” if you want. They are just names for a configuration sets that you can modify and you can tweak them so that for example the “Debug” configuration is not suitable for debugging any more. So if you (accidentially) disabled generating debug symbols in your “Debug” configuration you would get exactly what you described.
Go to “Project” -> “Edit active target”. Select the “Build” tab and select the “Debug” configuration in the drop down box on the upper left. Select “Show all settings” in the drop down box below and type “debug” in the search bar right next to these two drop down boxes.
There should be a setting “Generate Debug Symbols”, make sure its check box is set. “Level of Debug Symbols” should be set to “All Symbols”. Also, the “Debug Information Format” should be set to “DWARF with dSYM file”.
After changing the settings, clean your project (“Build” -> “Clean”) and build again.
Hope that helps.