Laptop fan question

I am working on wiring a fan into an older Dell laptop. The stock exhaust fan died on me and buying a replacement from them is too expensive for my budget ($100 for a new cooling assembly). I checked the voltage output from the header on the laptop motherboard and purchased a corresponding 5v fan (30mm) and I have it correctly position and wired, but the fan does not spin up. If I connect it to another 5v power source such as the 4pin molex red and black wires of a power supply, the fan spins, but it will not when getting the same voltage from the laptop motherboard. Any idea what is going on?

Thanks for any help.

Post is geting a little old, have you fixed this problem yet?

There are a few things to check to determine what is going on, happy to try and assist you through it if you are still working on it..