I suspect when you called semilogx as you said in your answer that probably should have been a comment you plotted on a linear axes first (with plot), turned hold on, then tried plotting a second time with semilogx. If you do that, hold will prevent semilogx from making the axes have a log scaled X axis.
To avoid this, either don't try to plot two graphs on the same axes with hold on but instead plot just the log scale data or change the X axes scale to 'log' directly, without calling semilogx. The hold function will prevent MATLAB from automatically switching the scale, but it won't prevent MATLAB from switching the scale if you explicitly order it to do so.
This example will not switch the X axes scale.
h = plot(1:10, 1:10);
ax = ancestor(h, 'axes');
ax.XScale = 'log';
This will too.