Try to reduce the volume of Lipofectamine added. If your cells are really sensitive to Lipofectamine( I ve experienced that before), try to use another reagent which is less harsh such as FuGene HD.
5-10% transfection is really low for Helas. Using L2k i was able to achieve minimum of 60-70% efficiency with no optimization. It is vital that cells are mycoplasma negative, i have seen its effects on transfection efficiency in myco + cells.