low volume on repress 7"s?

I been buying alot more 7"s lately, lots of repress issues of old roots. Seems to me that alot of them have very low volume when played on a decent turntable. Anyone know a possible reason for this?

Oh, the why is I'm going to DJ out in April after a 20+ year break from spinning for an audience. You know they didnt even have CDs the last time I did this in public and now thats all most people in Tokyo use. I feel old.

Update: There was a thread on Blood & Fire which related to this. The general consensus was reuse of old stampers as the cause.