Thanks again everyone. I'm still not convinced. I won't argue that my negatives aren't thin and/or underexposed. But the print I got of the boy (#1) has much more contrast than the prints I'm producing today (from similarly thin/underexposed negatives).
I bought my Ilford multigrade developer (PQ) 13 months ago, but it has always been kept full in an airtight bottle. I wouldn't expect it to be bad, but I might pick up a fresh supply anyway.