QuickBooks inventory is date sensitive. In addition, the build transaction requires that you have enough component parts on hand to issue the build.
If you issue a build transaction, then make some sort of change that decreases the quantity on hand of a component part so that there isn’t enough on hand on the build date, QuickBooks will change the existing builds to pending.
This can be due to an inventory adjustment, or a change in another build for a subassembly.
Remember that QuickBooks is very senstive to dates, so make sure that the dates of your adjustments (and builds) are correct.