Like the other folks said, its possibly heat or too much water..
How often do you water?try leaving it to dry out a lil,and water only when the plant looks thirsty, its much easier to remedy an under watered plant as opposed to one that is watered too often..you wont be getting vital air to the root system and can cause root rot/mold.
Get the fan positioned so it takes the heat away from the plants, yet still allows the leaves to have some movement, to toughen up the stems a lil..
Also if the stem is 2 1/2" i'd be inclined to say try getting more light in there,maybe get one of those cheap 20w CFL bulbs (equivalent to 100w incandescant) in cool white or DAYLIGHT if possible, get the Highest KELVIN rated bulb you can find,Typically 6500K is ideal....just check the packaging..
Cuz its prolly not getting enough light off the single 60w(is it a Regular Light bulb???Cuz that wont work).. you can buy clip-on light fixings for a coupla bucks, and get it 1 or 2" away from the canopy.
Remember, the more light she gets the faster she'll grow..
I've got 3 x 20w Daylight 6500k cfls and 1 x 20w Warm White 2500k cfl on one plant,no heating pad, 15 days old and has tons of leaves,with more growing daily,and the main stem is only 1 to 1 1/2" long..
They grow more each day approx 10-15cm..
heres a pic..
Hope it all goes well with those potential bitches..
