I used to get a very high yield of plasmids with Promega maxiprep and miniprep kits, but it started to fall to +100ng/ul or 50ng/ul, respectively, for maxiprep or miniprep. I suspect that the silica columns are just not that good as they used to be.
The binding of the DNA to the columns is pH dependent. Although, I have a feeling the reduction in purification is due to the bacterial cultures and not the columns. I would suggest re-transforming your DNA into competent cells (specific for DNA amplification like XL10 Gold) and then grow cultures from several of the newly transformed colonies. Pick the colony that gives you the best yield to make a glycerol stock. I hope that helps, Good Luck.
Yes, the cultures should not be overgrown otherwise the plasmids will be degraded and your yield will suffer, alternatively too little growth you will also not have high yield. For spin minipreps, we generally inoculate 5mls of LB broth and grow for ~16hrs.
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