Have you installed the
patch? An earlier version had a bug which could update the quest log prematurely if you fond the alchemist's ingredients before talking to him, picked them up and moved them in your inventory. This in turn would prevent the cut scene at the rock which opens the way to the cave below.
If that is the case here;
[*] Complete all other quests and save the game.
[*] If you have a save before finding the alchemist's ingredients and moving them, copy the 2 story.* files from that save into your latest save, otherwise you can use
these files from a save at the beginning of the imp village.
[*] Talk to Altoflix in the north of the village (if required), then the chieftain's son to get the quest to find the alchemist, talk to the alchemist once (only) and head to the cave.
[*] Continue with the game.