Nozzle temperature hysteresis?  

Nozzle temperature hysteresis?

I'm trying to print a temperature tower. I've inserted the g-code to change the temperature at the appropriate layers, and I see the set-point change to reflect that, but the nozzle temperature doesn't change unless the delta is fairly large.

My print starts at 215°C and the g-code should bump it to 225°C for the first test, then step it down from there. The display shows the set-point change to 225°C, but the temperature was stable at 215°C for over a minute after. I tried manually bumping the temperature to 226°C, and that also had no effect. Then I bumped it to 240°C and the temperature immediately started moving up. When it got past 225°C, I reset the set-point back to that temperature and saw it stabilize nicely.

Is there any hysteresis on the nozzle temperature control? If so, what are the hysteresis limits? Do I need to do more than put an M104 in the g-code to make the temperature actually change? Would it be sufficient to put two M104s back-to-back: one high enough to escape the hysteresis, the second at the actual desired temperature?