Custard is very damp;Custard get down to wycombeyou prob not used marker for a while or it been kept somewhere slightly damp
so what they said above clean it and use a very little dab of oil on it.
dont forget a little dab el do ya same happens to mine when i decide to take it out for a day which is not often anymore as im likin me shocker , ion ,and pmr....sooooo much lighter and smaller