Don't forget PSU's don't put all their power on just the 12v line.
So 300w is not like you can connect up to 300w of equipment on the same voltage.
Besides that, usually the indication on the PSU is a peak and not steady (much like watt on a speaker, 1000w peak, 20w RMS)
Just a random image I took from google:
http://www.dansdata.com/images/top686p6/sticker800.jpgYou can see here it's a 580w PSU but it only has 420w on 12v
Just try them 1 by 1 and see if they work. If you hook them all up and they don't work, it's quite assumable it's your PSU