I'm having a problem with my miniprep yields. Usually I get 100-300 ng/uL with a 1.5 mL culture using the Qiagen kit. I do plasmid prep frequently, but recently I've been getting around 30-50 ng/uL all of a sudden. I've tried doing all the troubleshooting listed in the manual (extra wash step, incubate elution buffer, use more bug culture, etc.), using a brand new kit, fresh antibiotics (and testing multiple plasmids with different resistances), even using Macherey-Nagel's Nucleobond miniprep kit. Other users in my lab get the same results, so I don't think it's something I'm doing wrong technically.
My maxipreps, on the other hand, are great and I consistently get over 2 mg from a 500 mL culture (from the original miniprep culture that I can't seem to get plasmid from using a miniprep kit). I can't afford to maxiprep everything though, so I'm trying to figure out what's going on.
Any thoughts or suggestions would be greatly appreciated!