I can't get drops from mobs in all slots. Even if I progress according to my level, no drops seem to drop. Has anyone encountered this error or problem?

Problems could be:

1) you have no active pets (min. 1 )

2) you have hungry pets -> if they have no food, they don't collect anything from enemies (they have no strength)

3) you have exceeded the inventory weight limit -> you can't carry (get) any more items unless you move the excess material from the active character's inventory or transform it at the "Trader" NPC -> materials obtained from the Great Desert or the Eternal Frost (cannot be moved)
or you don't sell excess material to one of the NPC merchants