It looks like you might be using a stackedplot which currently doesn't support adding objects to the axes after the plot is created. You can, however, plot multiple lines in each axis (see this answer).
Subplots and tiledlayouts do allow adding objects to the axes.
Option 1:
use hold(ax,'on') to hold the axes 'ax'.
use plot(ax,____) to plot on axes 'ax'
Option 2:
Copy a plotted line 'h' to axes 'ax' using copyobj(h,ax)