I don't get rare items
In order to get a valuable drop, you need to fight monsters with a level no lower than 1 below your own level. In that case, you have a chance to get a rare or valuable item.
The choice and distribution of the dropped items are random. The likelihood to receive a rare drop (e. g. green or blue items) is always the same and will NOT increase with the number of killed monsters. It may happen that you kill one single monster and receive a rare item or you can kill thousands of monsters not receiving anything. In both cases, there is NO error.