I am trying to edit a user preset in Nova System. For example, I want to add compression to an existing preset. If I press the Comp button I can edit the parameters, but I cannot hear the difference and after storing, the compression is still not active.
When you press one of the blocks button, you will enter the edit mode and access the parameters, but the effect is not automatically activated. You need to use the corresponding footswitch to control the ON/OFF status for the selected effect. In your example, press the COMP button first, but also press the COMP footswitch to activate it - Then edit parameters and store - at the next recall you'll see that compression is ON.
Note that the EQ/GATE block has no corresponding footswitch and include a ON/OFF parameter in the menu.
Also note that even if your board is set in PRESET mode, as soon as you press one of the edit button, it will temporarily switch to "pedal mode".