PWM circuit

I was wondering about a problem I have with the switched constant current source at this site http://www.hoelscher-hi.de/hendrik/english/led.htm The circuit is the one around the bottom of the page to the right. I tried it and it works well when I use 12V input but when I try 24V input on the LM317 using a string of LED's they stay on and the PWM from the ATMEL does nothing. I have 6 led's in series which should be around 18V. I put a 10K resistor from Gate to ground and there is still a faint glow on the LED's. Thank you for any help on this.