I hear nothing but the piano

Hello everybody

I have a little problem tough I think I am the problem ;). When I try to write something for orchestra, I can only hear the piano: all the others instruments are silent. i think there must be some parameters I changed without meaning it but I am a bit confused for I only hear the piano when I write for six or seven violins and a clarinet ... When I just write for the piano and I take the piano scores, it's all right but when I add the rest ... nada.

Thanks a lot for your help