Weed (a contributor to this board) has his own thread that he started from this reciepe and swears by 225 overnight. you want to keep the temp below the vaporization. Basically if you get it to hot your releasing the thc into the air and not the oil. According to the people at NORML the vaporizing temp of Gods greatest herb is around 180 to 190 c 356-375 f. Try 225 and try not to let it get over 275-300. Also how are you grinding/chopping your weed. You want to get your weed almost powder consistent for the best results (The more surface area in contact with the oil, the more trichomes that are exposed to the oil) In regards to the potency you have to be careful not to use too little oil for the weed. Flame (the creater of this post) has found that messing with the ratio can cause problems. The ratio that apperantly works is 1 1/2 teaspoon of Coconut oil to 1.5 Grams of dank. I wouldnt try this reciepe with anything less then mids. Its a vital indgredent.
The 4 T's to THC extraction IMHO
1. Temp - 225-275 try not let it spike over 300
2. Time - Let cook overnight
3. Texture - Grind Grind Grind to as fine a powder as you can
4. THC Content - Dont use anything less then mids
5. THC Ratio Don't use to much herb to oil. (I f i want to try something stronger i would just take a few extra)
Hope that helps