Is it xenons you have on your cup?
The starters are fiddley, inside you'll see a plastic ring that rotates slightly, along the ring there will be notches in it, and you can see where the notches on the plastic ring, have to line up with the notches that let the bulb into the starter.
The number of times I've pissed around trying to get the starters back on, and then realised the plastic ring was out of position, is rediculous.