My Citroen C2 occaisionally fails to start?

Citroen C2 (1.4 furio) occaisinally fails to start..... starter motor turns engine over but plugs don't catch..... when they so then revs are very low until you 'blip' the throttle.... Anyone any idea?

Note that it can go days/weeks without doing it then has a oeriod of consistently doing it !