Since the XTick of the 1st, 2nd... box plots are 1, 2..., x = 0.3 might be too small to display on the graph. If you adjust (x,y) value of the dot properly, you can add the dot on the plot, like:
x = rand(1000,1);
group = mat2cell('a':'h',1,ones(8,1));
g = group(randi([1,8],1000,1));
boxplot(x,g)
hold on
scatter(1.5, 0.5)
