Question about S7 debugging

Suddenly, I cannot use breakpoint anymore, there are only "Delete All Breakpoints", "Breakpoints Active(on/off)" and "Resume" is not gray. What should I do?

Siematic manager view=online, block editor debug=test, cursor must be on a line of code, and you must have your block editor view set to STL. (In ladder view, you may still see the code in STL, but the breakpoints will not be allowed)

Thank you, Simon.