It could be a capacity issue - from what i've seen, if a stockpile can still accept new bins, the dwarfs will assign _one_ new bin, then fill it, and only once the most recently added bin is full a new bin will be added. And they can only fit five bars into one bin, while with other types of items, you can have hundreds in a single bin (notoriously the case with cloth). So the bar stockpile goes through - assign a bin - wait until five "stuff items in bin in a ridiculously inefficient manner" jobs were generated and executed - assign a new bin. Especially when the feeder workshop is a long way from the stockpile, those jobs take forever and a day to execute and you can get very long pauses between jobs.
Use of "active" bins is also more likely to happen when items are shifted in and out of stationed bins - this happens a lot on clothing stockpiles through claiming of fresh clothes or on leather/cloth stockpiles when materials are removed for crafting and free capacity for new storage jobs - but will typically not happen on soap stockpiles, so the "old" bins are fully dormant and only the newest bin sees any activity. In my experience, bin-accepting bar stockpiles eventually stuff their contents into bins, but it takes quite a while.
^The only stockpile types which automatically have a wheelbarrow assigned are stone stockpiles. You can assign wheelbarrows to any stockpile you want and set the number of wheelbarrows reserved by hitting the "w" key while in the stockpile's "q"uery menu.