what is that on top of your magnet? It looks like there is something there that could be removed and is probably just adding extra wieght (which may be part of your problem). I have six DIY stir plates that I made all with 80mm fans and hard drive mags just like yours. Sometimes, I have trouble getting them to start while the starter is sitting on top. I think this is becase of the extra resistance that the stir bar adds to the mix. What I do when I have this problem is take the starter off the plate, take something metal and move it around on top of the plate so that the fan spins a couple times and try again. Sometimes I have to do this more than once but I can always get my plates going eventually. All my fans start right up when there is no starter on top. Also, how much input power are you using? I need at least 9 volts to get mine spinning and then dial them down to around 4-6 volts for normal operation. I use a variable voltage regulator and a pot so that I can adjust my power from 0-12 volts.
