Well very little info, so very hard to say.
You say in other projects it works?
So I guess you have "GDB" as debugger selected in: Menu: Environment => Options => Debugger ?
And you have set the correct Path to gdb.exe ?
How about your Project setting? (Menu: Project => Project options => Compiler Options => Linking) (older Lazarus "Compiler Options" are in the menu).
Do you have "Debug Info (-g)" OR "Dwarf debug info (-gw)" enabled?
Where is the Breakpoint in your project? Or in the LCL, or RTL?
When you project runs, do you get blue dots in the editor's Gutter (next to the line numbers)?
What is the output of: Menu "View" => Debug Windows => Debug Output ?