I'm unable to reproduce your results. Obviously I don't have the starting dataset (hyd3D). but if I try to produce a meshc with a dummy dataset, and then follow the meshc with your annotation commands, I get results that show the axis labels.
[X, Y] = meshgrid (linspace (-3, 3, 40));
Z = sqrt (abs (X .* Y)) ./ (1 + X.^2 + Y.^2);
meshc (X, Y, Z);
xlabel ("Delta x",'Color',"k",'fontsize',10)
ylabel ("T (K)",'Color','k','fontsize',10)
zlabel ("Diff. (kJ/mole)",'Color','k','fontsize',10)
What version of Octave, operating system, and graphics toolkit are you running? (type graphics_toolkit at the command line). Can you try the plot using other toolkits?
and see if you get the same or different results? On my system (Octave 6.2.0 on windows) i get the same results showing axis labels using all three with all three toolkits, but it might help us determine what's causing your error.