The small value range is what makes it flat, you also tend to use a light and pastel look which would be around 2-ish I guess. (see image below)
To make the image pop and seem 3D you want to be using a large range of values (for example from 1 through to 5 in a realistic or portrait piece), from really dark (shadows) to really light (light saturation) and then halftones (Value 3 and there about) to shade the areas where light and absence of light mix. I would say you use 2-3 which is only a change of 2, however you are looking for a change of 3/4 maybe even 5 if you were going for realism.
1 >>--> 5 (changes from artist and range used, for example 1 >>--> 10 etc)
Dark to Light