Cistern not filling unless press button repeatedly

[spursfan323]

Cistern fills very tempermentally- occasionally it fills normally, most other times have to press button repeatedly to kick into action (then fill 'normally')

Does it sound like the filling mechanism alone is broken, or is there another cause?

BTW brand is banyo (I think may be a copy/second of Bagno)

 

[flameport]

Some part is probably sticking or jammed with limescale.
Take the lid off and look inside.