I am using SW 2016 SP 1 - I had this same problem with 2014 and 2015.
When I add a part or sub-assembly to an assembly, then go to the drawing to see the items are added.
If the part or sub-asm is not listed, i try and added to BOM - Insert row. the part is not listed even if I try typing the info.
Why is the item not listed? I have to delete the BOM and re-enter. - this is a non-productive fix.