You need differences in shadow heights, to make the isometry more believable. Higher objects cast longer shadows. Try it, you'll see what I mean right away.

All your edges which face upwards have very little definition. In real life, we recognize edges on a glance by a change in contrast between one surface and another. Your contrast (change in tone) is very slight between your top grey and bottom grey. Add to that the very fine brick detail -- I would emphasise that a lot. Also not that gaps between bricks aren't just a dark shade, more often than not you'll see highlighted edges contrasted directly adjacent to deep shadow. You have zero highlights -- of course things will look flat. Check this out as an example:

(from here) And yes, I know it's perspectival and not orthogonal. The point is the lighting.

You need differences in shadow heights, to make the isometry more believable. Higher objects cast longer shadows. Try it, you'll see an improvement right away.

All your edges which face upwards have very little definition. In real life, we recognize edges on a glance by a change in contrast between one surface and another. Your contrast (change in tone) is very slight between your top grey and bottom grey. Add to that the very fine brick detail -- I would emphasise that a lot. Also not that gaps between bricks aren't just a dark shade, more often than not you'll see highlighted edges contrasted directly adjacent to deep shadow. You have zero highlights -- of course things will look flat. Check this out as an example:

(from here) And yes, I know it's perspectival and not orthogonal. The point is the lighting.