pET14b vector with insert transformed in BL21(DE3) RIPL cells - (Nov/05/2012 )

Hello Dear
Hoping you will be fine. i am using BL21 Codon Plus RP for expression of my protein using pGEX-KG Vector. Because pGEX-KG is Ampicillin Resistant so i use Ampicillin+Chloramphenicole Plates to culture the bacteria but when i transfer the colonies in Ampicillin resistant LB broth or the broth which contain Ampicillin+Chloramphenicole then the colonies not grow in broth. Colonies only grow in Cloramphnicole resistant broth. Then i have checked my plasmid using PCR but all colonies are negative (colony which grow on Chloramphnicole resistant broth). Please guide me in this issue
Thanks