Insert mode is voice-specific: it’ll add and remove time for stuff that’s voice-specfici (e.g. notes) but not for stuff that isn’t voice-specific (general dynamics, tempo marks, fermate, time signatures etc.).
If you need to add or remove time globally, do so using the Bars and Barlines popover, or via the System Track. You can, for instance, add a minim/half to a single staff by placing the caret on that staff and typing Shift-B 1h Alt-Enter, or two bars by typing Shift-B 2 Alt-Enter.
P.S. There are plans for Insert mode to work in different ways in future, but not yet. See Insert (I) bit me again - #28 by dspreadbury for instance.