It was fitted carefully last year. It never ran smoothly then. I tried a bit of force for re-aligning at time of fitting.
Will try sanding edges more and may be twisting for re-alignment again.
Twisting did not seem to help last time.
The mix of the plastic track 'joiner' and the aluminium rail seem to make twisting for re-alignment almost completely ineffective. The Al H-section does not seem to budge much, and the plastic joiner does not seem to deform in an expected way. Exasperating.