I have the same problem with my new bonsai.
I got a bonsai for my birthday two weeks ago, and it was lovely and green and healthy looking. Now, half the leaves are yellow and if I brush against it, there is a shower of dry, brittle leaves from the tree.
I water it every day, and never let the soil dry out.
I'll try the checks Oirish. Since it's only 2 weeks old, I dont think I should have to repot it yet. Is it important to leave it in sunlight? I have it in the shade, since I dont want it to get roasted.