Problems with Transformation / DH5-alpha / Plasmid DNA

We have cloned a gene in vector pVITRO2 (GFP-HYG-LacZ; 10KB), and transformed it into DH5-alpha E.coli cells with suitable antibiotic (of course in the right concentration). The problem is, after incubating O/N at 37 deg C, we notice a few colonies on the LB-agar plate, and with great difficulty if we are able to locate, and inoculate a single colony into LB for Miniprep, at the end of the procedure we do not find any plasmid DNA!! (on 1% TAE agarose ge). Can anyone give a solution for this problem?