It's also possible I've given bad advice. Let's see if anyone else can confirm/deny what I said.
By the way, you need to be much more specific than "doesn't work as well with turning on the fan" before anyone can help you. What, exactly, is it doing? And what, exactly, were you expecting?
If you've got access to an oscilloscope you can observe the PWM waveform as you move the pot. If not, then println() the value of val to the PC so you can see what it is doing.