Yes, the modulation section doesn't automatically do anything without a mod wheel. If you are using a MIDI controller with a mod wheel, you'd probably never notice this because it's intuitive to use a mod wheel to turn up modulation. Since you asked, I'm assuming you aren't using a MIDI controller with a mod wheel.
I added a special trick to the Slim in order to deal with this:
Press and hold the MOD AMOUNT button for half a second or so and it should start blinking. You're now in "mod wheel" mode - the Modulation knob is now a mod wheel.
Turn the modulation knob all the way up. Now the modulation will be fully active, at whatever amount you set using the actual modulation Amount parameter.
You can press and hold the MOD AMOUNT button again to exit this mode and adjust the normal Mod Amount parameter again.
Once the "mod wheel" parameter is at maximum, it will stay that way until you change it yourself or until you power off the synth (it always comes up with mod wheel at zero). Or of course if you send a MIDI Mod Wheel message it will override this setting.
Hope this helps!