What is your humidity? I had this problem when my basement humidity was
below 25% I now have a whole house humidifier that keeps my humidity at
50%. Your media type dictates how often you should water. What kind of
growing media are you using? Feeding to often can cause growth that is too
rapid and sometimes weak. Also are your roots healthy all the way down to
the bottom of the pot?

I also water twice a week but I feed only once a month. My growing media
is a mix of coconut husk, charcoal and perlite.