I really can't imagine what, other than debris, could cause this to happen James. The complete cure for all time is to replace the chuck with a MUSCLECHUCK, it makes bit change from the top even easier, plus only a 4mm ball ended Allan key is required to lock/unlock the bit and it is held with a grip several times that of a collet chuck .
As a matter of interest, if you turn to page 5 of the TRA001 hand book, you will see that it reads: "NOTE: When fitting cutters, ensure the shank is inserted fully into the collet"
Nothing but heaven itself is better than a friend who is really a friend. - Plautus