This issue only appears sometimes! So it is difficulty to reprocedure.
If you try to pick of items, which where in the inventory of another player before, than some items can't be picked up. Some items can be moved in your inventory other items not (there is nough space in my own inventory).
This issue appereas often if you try to pick up items from dead players or containers which where filled up by other players.
This issue appereas sometimes if you try to pick up items, another player has dropped. In this case the dropped item is also not visible, sometimes.
This issue looks like a small problem, but it will cause a lot of frustration if you collect some loot in a group and want to help/trade with other people and it will not work because some items can't be changed. So this issue is a major error.