Any mellophone player can you please give me some advice?

Hi there, I am a sopre and in marching band. My main instrument is trumpet, but was switched to French Horn in December. For this season I have to play mellophone. It is very hard for me because there are only three altos and one other mello, but he doesn't play, he just learned the instrument. I am always getting yelled at since I'm never heard, any tips to play louder or how to stabilize better would be greatly appreciated.