Along with what Manic said. Something is yielding and relieving the clamping force. Could be gaskets (use only factory gaskets if you can), The bolts themselves are torque to yield bolts, that means you torque them, then add 180 degrees of rotation to stretch them. These types of bolts are typically one use only in my experience (automotive). Perhaps yours are shot, bad temper (from the prior meltdown) or re-used too many times. You confirmed you used the proceedure in the manual, did you put the moly grease on the threads and the washers?. The washers are there and are correct? Are the threads OK. Have you checked your head and cylinder deck for flatness?
I would get the head and deck checked for flatness, check the threads in the cases, buy new OEM gaskets, bolts and washers, a tube of moly grease. Only about 10% of torque is transfered into clamping force. The rest is absorbed in head and thread friction, which is why the moly grease is required.