do you have a fan blowing air on them to strengthen up the stems?? i agree your light could be to far away so your plants may be stretched.. you need your light as close to the plants as you can get it without burning the plants.. but also get a fan blowing air gently on them so it strengthens the stem.. it strengthens it by causing little microscopic breaks and fixing them.. over and over... which makes it strong.. anyways.. hoped this helped...
good luck
sampson