I have NEB 5-alpha Competent E. coli transformed with my plasmid that has a KAN resistance gene. Although the cells are growing very well on the KAN agar plates, they are not growing well in the LB broth. I tried the same stocks of the agar plates and the LB broth with other plasmids and they are growing well. However, with this plasmid, I either get very low growth in the LB broth or almost nothing. So what can be the reason for that?
Check the incubation temperature and also make sure the LB broth is fresh.
No growth on LB plates suggests you have no viable cells in your SOC medium. Colonies on Kanamycin plates are most likely contaminants. Check your stock kanamycin plates(if you have any stored ones) to make sure there is nothing growing on it.