Current monitor for fan running with PWM through FET

I tried this technique but without luck. The same problem like before. If i am running FET with 255 (100% PWM) from arduino signal led is always on but if i use any other PWM value (191 for example), led is always off (also if i am holding fan).

Any idea?

I am using TC670 with IRF530 FET.

TC670 scheme

FET wiring