OK, so whats the difference between a mixer fan and the computer fan I have??You may have to add a small mixer fan.
the cold spot is on the far end of the bator, away from the heat source. I have had great hatches in this bator, but never 100%. Maybe this is whyA cold spot sometimes indicates faster air movement in that area so reducing the fan speed by reducing the voltage may work. I normally use a 6 to 9 volt power supply to run 12v computer fans as this slows them down which makes them blend/mix the air rather than blowing the air.
If the cold spot is further away from the fan then this can be caused by a over sized heater (normally this will balance out after 30mins but may be noticed if you are opening the incubator to move the temp probe around)
if you have pictures of your set up and the area that's reading low we can give you better advice