Loss of compression

I have a 1971 BMW model 2002 and the car has been over heated quite regularly actually my brother was driving it to work and was overheateng it almost every day. So anyway long story short I noticed a sudden loss in power. So naturally I checked among numerous other things the compression. Well they were all low I believe less than 30 psi. Now this didn't make sense because I have put a set of high compression pistons in it. So I replace the head gasket and found out that the old one had in fact been leaking. I also checked for slack in the valve train and it is all within specs. However when I put the head back together and tested for compression again it is still low. I got between 30 and 45 unbalanced compression. So what should I check next?