From my experience, mobile Android users using 2.1 or lower have some issues with posting in shoutbox, it's not a you
thing, so don't worry. If you can, continue to post your shoutbox issues and your phone specs, even though some may already have. The more we speak up about this, the better!
Until then, I'm sure you want to post in shoutbox! Here are some of my suggestions!
My first suggestion would be to update your phone to 2.2 (FROYO) or if you're really fiesty, 2.3 (Gingerbread). I know AT&T was pushing 2.2 for their android phone Captivate through mobile text. If anything, contact your carrier and see if they are offering downloads. A simple google search will let you know of your options.
If you're not into updating your phone's software, that's okay, because Mozilla's Firefox web browser allows users to post in shoutbox. To download, search in your App Market for Firefox or use your QR scanner to navigate and make sure you are using the correct app.
(It is my assumption if you are using a smart phone, you should be able to naviate some of what I am suggesting, if you need help or direction, I am willing to help through PM.)