I have followed following steps and was not able to reproduce this issue.
1. I went to "Photo" section of my site
2. Clicked on random photo
3. In the opened window I clicked on "Comment" section
4. Form where I can add comment appeared with "Add" button at the bottom.
Suresh, what mobile phone and browser were you using? Please be informed that even though we did not optimize Oxwall for mobile devices, we did everything possible to make it work properly in Safari on iOS and in default Android browser.