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Not being able to reply can be caused by any number of reasons and whilst we try as much as possible to make the site accessible to all sometimes there are things that are outside of our control.
I don't have a tablet running android so can't test the site out that way unfortunately however if you can give me some more detail then I can go take a look and see what might be the cause and , if possible, put a fix in place.
So a few questions /comments:
Can you give me a rough date / time when you tried to reply to a post
When you open an old discussion do you get the "Leave a comment" box displayed with the buttons to "Save Draft" and "Post Comment" at the bottom
Are you using the in-built browser on your tablet? From what I can see although the browser is considered quite good it's not as "feature-rich" as others so it's possible it might not be able to render some of our html
What version of the android o/s are you using? If it's above version 4.0 have you considered downloading Google Chrome
Have you considered / could you try downloading Firefox
If you can give more information I'll happily take a look. No guarantees of being able to solve your issue of course but I won't know until I try