I had this issue as well. What I think it was was that there were just too many items in the containing space, since removing a lot of low-volume items from the area allowed me to fill the area up to the volume limit with a smaller amount of higher-volume items. I recall watching one of Vorm’s videos a while back stating that there is an upper limit to the number of items that can inhabit an area, regardless of volume limit, but I don’t know code or anything like that so do your own research.