if I remember right co2 does allow for higher temps but if it's gettin over 100 in there I'd agree you should start there and work once that's resolved

also depending on the type of strain a 400w hps won't grow huge like show buds especially if it's not extremely close to the plants 400 loses power quickly as you move it away that was definately a problem of mine on the first grow

a pic always helps give us some of the info if ya know temp and light distance maybe ph and ppm foods plant height how long in 12/12 etc