I have been trying to perform mini-preps w/ 5 ml of culture trying to isolate a group of pGEX plasmids. As well as doing this I have been running a positive control with a plasmids that I know works. Everytime I have performed this prep I've gotten very low yields while my positive control is still working. I was wondering if there could be an explanation as to the nature of the vector or possibly that my bacterial stocks in the -80 are just old. I've tried it with BL21 and TOP10 bacteria. Any suggestions would be appreciated thanks.