it sounds like the hps is too close to the plants I use Compact Fluorescent in a silver shop light till they are 2 to 4 in high then use the hps or I have also done is to use
Fluorescent till time to flower then swich to hps doing this I get more buds depenting on what kind iam growing "big bud works great this way" there is also Fluorescent work lights that I have used and works great I have even been able to do a whole grow with it but useing hps for flowing is even better

hope this helps