toilet cistern does not flush?

My plastic toilet cistern, which I believe to be single syphonic, fills up with water ok but handle depression no longer causes a flow of flushing water. Prior to this two depressions of the handle were usually needed to get flushing water flow.
Is a Spring Bathrooms B5 British made pan fitted with an SRC green cap on the top of what looks like a syphon (a vertical twin tube; one bottom rising vertically from (and being part of) a wider plastic container that the handle action draws a plunger vertcally upwards through. The other twin tube routeing flush water down to the toilet bowl.
Any ideas please on likely cause and how to approach repair work?
Many thanks,
Henry Hall