I also made a simulation in LTSpice for this schematic.
Please have a look...
The frequency with C4=470pF is about 50KHz, and with C4=1nF, F=24KHz. Which is the correct frequency for the DC fan motor (120 mm fan) ?
I checked the physical circuit, and it does not work with the 120mm fan. The fan does not spin. I also checked with 2 80mm fans, one by one, and it is working. Do I need a particular type of 120mm fan ?
The 120mm fan is rated at 12V and 0.16A (DC brushless fan). I don't know the current rating for the 80mm fan, all I know is that it is rated at 12V.