It takes a while some times...I never really figured it out. I'd just be randomly playing along and all of a sudden get an item.
It is annoying, because as much as people will say the items don't make it an easy win I would disagree...well, in some cases. For example, a heavy with a weapon that increases the rate of fire is a mean machine...even if the gun reduces damage. The higher rate of fire more than makes up for it.