Try choosing a more interesting light source. Your light's coming from the front, so the tree has no volume because it has no shadows that can show its form. Try putting the light above the tree, off to the side, etc. Anywhere but directly in front of or behind the tree should work.
Your colour ramp is uneven. You have three light greens that are all somewhat similar (the contrast between them's acceptable, but not high), then you have the two dark greens that blend together a bit, and nothing inbetween. Try darkening your mid-green and lightening your 2nd darkest green, so that your colours cover the value scale (dark-light scale) more evenly.
The browns on the trunk have a similar problem as the greens, there's a huge step between the light colours and the dark colours, making the trunk feel rougher than the pixel work would otherwise suggest.
This kind of contrast (lacking midtones) can actually work quite well when done intentionally, giving a somewhat noir-ish look with heavy shadows, but to do it well you have to be very attentive to the boundaries between the lights and darks. I am guessing that's not the effect you were aiming for, though.
Your leaves actually look alright. When working with a limited number of colours and a low resolution, the main tool you have for showing detail is the boundaries between the colours. Keep that in mind as you pixel. It's okay to lose details that are inside of your clusters (connected areas of a colour) - chances are, you don't need them or can't fit them anyway. Focus on suggesting the leaves, bark texture, etc with the edges between colours, rather than drawing out the details in full.
