What are the chances of getting the trousers from the hefin agility course? According to the runescape wiki they are a common drop, whereas the shadow cores and lamps are rare, I'm yet to get a set of the trousers but I've had multiple cores and lamps?
The trousers should still be "common". I do not think there has been an update in the recent months to change that. I'm afraid you are just unlucky (or well, lucky in this case) for getting all the other items before the trousers. :/
Remember that the information on the wiki might not always be 100% accurate, as everyone can edit it and the rarity is determined by a certain amount of trial laps done. There could be a piece of code in place which impacts the rarity of the item, but that we do not know off. The wiki simply guesses (and mostly does this correctly), but we will never exactly know what the rarity is until Jagex confirms it.
If you keep trying to get it, you will get it eventually, however! Sorry I cannot give you a better answer