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You do not mention any other part of your system. The most likely cause is that there is no source of a clock message. And without a clock message time is not advancing as as such your timer callback will never be invoked.

You can read about the design of simulated time here: https://design.ros2.org/articles/clock_and_time.html

The most common sources of simulated time are a simulator like Gazebo or rosbag when playing back data.