I agree with most that they just don't seem to have the ability physically or mentally to play the man to man defense Jamie prefers. Also, we know/hope Durand will be playing a lot of minutes next year, and defense has never been his strong suit.
Unless they make huge strides over the summer, Jamie is going to have to come up with a different defense they can succeed at. 2-3 zone didn't look too good, but there are lots of other zones that can be learned relatively easily and some of the guys may have played in high school. There are also other options for man to man and guarding screens, etc. Jamie just needs to find what works for this group. If he insists on sticking with this man to man, I don't think we'll see much improvement.