No PCR product visualized using quikchange kit - (Sep/26/2008 )

Another strange situation is.....that after electroporation of my PCR product in to XL1-Blue E.coli....Some colonies appear on LB+zeocin agar plate...however, when I transfer the colony to 5ml LB + 25ug/ml zeocin broth, no bacterial growth occurred....

I have checked the LB broth (can promote E.coli growth when there is no zeocin) and my LB+zeocin agar (no growth of E.coli which have no plasmid)...how come is that...?