Calor gas fire. Why doesn't the middle panel light?

Gas fires need servicing sometimes to remove the natural build up of dust, fluff and carbon and it is probably this that is causing your trouble.
Service engineers will remove the radiants and vacuum the burner(s), the mixing chamber and check the gas jet(s) are clear, the gas pressure is correct and there are no leaks.

Using a vacuum creates less mess but it doesn't always get to the heart of the trouble and many fires that have been condemned as being worn out often need stronger medicine - should fluff be partially blocking the air supply to the mixing chamber the gas/air mixture will become gas-rich and the flame will create soot, sometimes lots of soot that vacuuming doesn't shift.
The careful application of a compressed air jet does tend to move lots of soot...

This is best done in the open air...