Your independent values (the indexes of y) only go from 1 to 5. Your first tick mark is at 10 so it is off the far right end of your plot and can't be seen. To prove it, and see the ticks, put this line at the end of your code:
xlim([0 70]);
To fix, and make somewhat more robust and versatile, you might try it like this:
clc;
fontSize = 20;
xTickLocations = 10:10:50;
yTickLocations = 10:10:70;
numberOfXTickMarks = length(xTickLocations);
% Generate sample data. 3 sets of Y data
% all of the same length.
set1 = [65 42 31 23 20];
set2 = [63 40 28 19.5 17];
set3 = [42 27 18 13.5 12];
xValues = 1 : length(set1);
% Now plot all three sets of y data.
hold on;
plot(xValues, set1, 'bo-', 'LineWidth', 3);
plot(xValues, set2, 'rs:', 'LineWidth', 3);
plot(xValues, set3, 'kd-', 'LineWidth', 3);
grid on;
legend('Set 1', 'Set 2', 'Set 3');
xlabel('X', 'fontSize', fontSize);
ylabel('Y', 'fontSize', fontSize);
title('Plot of Three Sets', 'fontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Ask user if they want to switch tick marks.
promptMessage = sprintf('Do you replace the default X tick\nlocations and labels with ours?');
button = questdlg(promptMessage, 'Replace Tick Marks', 'OK', 'Cancel', 'OK');
if strcmp(button, 'Cancel')
return;
end
% Replace default tick locations with ours
for k = 1 : length(xTickLocations);
xTickString{k} = xTickLocations(k);
end
% Make new tick marks.
xl = xlim(); % Get existing range.
% Make 5 tick marks. (Existing # of tick marks may not = 5)
xTickLocations = linspace(xl(1), xl(2), numberOfXTickMarks)
set(gca,'XTick', xTickLocations);
% Make new labels for the new tick marks
set(gca,'XTickLabel', xTickString);
