I agree with your observations.

After checking my log and return on 6 meter QSL's I have found that 6 meter
ops are the worst return rate I have. I do better on DX either direct, to a
manager or via the buro.
Have never figured out why it is so hard to get confirmations of 6 meter
contacts.
Don't know what else a person can do after sending a card with an SASE,
following that up a couple of months later with an email and another card
with SASE and still not getting anything back.