Heat Sensitive Fan Project not providing desired output to fan
What opamp are you actually using? The 741 is half a century old and is not very good by modern standards.
Also that circuit is not very good in that it uses a PNP transistor.
With the limitations of the opamp and the PNP transistor the fan will probably only get about 6V. The rest will be dropped by the transistor.
A better way would be to use even a slightly more modern opamp such as an LM358 (only 40 years old). And an NPN transistor or a MOSFET. The fan will then get a full 12V (minus a few tenths of a volt).
The only other change required would be to swap the inputs around and change R2 to maybe 470 ohms.
Adding some hysteresis by feeding back the output to the input would give a cleaner switching action and ensure that the fan starts properly.