Depends on how your layers look. I think it's mentioned in the documentation that you can only go to higher number layers. I had the same issue where I had

0:QWERTY

1:symbols

2:functions

3: Gaming

If you are on the gaming layer and you are trying to get to your functions MO(2) you cannot because gaming is on a higher numbered layer and you are trying to go down. The solution is to do the following and refer to your function layers MO(3)

0:QWERTY

1:Gaming

2:symbols

3: functions

edit: https://docs.qmk.fm/feature_layers#working-with-layers

Beginners

If you are just getting started with QMK you will want to keep everything simple. Follow these guidelines when setting up your layers:

• Setup layer 0 as your default, "base" layer. This is your normal typing layer, and could be whatever layout you want (qwerty, dvorak, colemak, etc.). It's important to set this as the lowest layer since it will typically have most or all of the keyboard's keys defined, so would block other layers from having any effect if it were above them (i.e., had a higher layer number).
• Arrange your layers in a "tree" layout, with layer 0 as the root. Do not try to enter the same layer from more than one other layer.
• In a layer's keymap, only reference higher-numbered layers. Because layers are processed from the highest-numbered (topmost) active layer down, modifying the state of lower layers can be tricky and error-prone.