Car: 9th Gen 1.4 Petrol SE + Navi
Join Date: 13th March 2017
Location: Essex, UK
Thanked 284 Times in 258 Posts
As it's xenons, it's an indication that either the bulb or the ballast is failing.
Swap the bulbs over, if the problem changes sides, buy new bulbs - If it stays on the same side, look for a new ballast!
This is caused by the way HID lights turn on; A high voltage (20kV+) pulse is used to start the plasma arc which produces the light; If the starting pulse is not strong enough, then additional strikes can get enough warmth into the bulb to start it (Though this is very bad for the ballast / bulbs!) or, if the bulbs are on their way out (They are two electrodes which are eroded by the arc) then the gap may be too large for the arc to start without the extra warmth.
Hopefully that sheds some light (ha!)