Backspace and Delete in vi.

I need to interact with a number of Unix/Linux servers at work. I use putty as an SSH client to do so.
I use vi as my editor of choice.
On some of these servers, when i try to backspace, or delete, text in VI, i end up with varied results. Sometimes when i press the delete key, it doesnt delete a character, rather, it capitalizes it. and backspace generally does nothing.

Both keys work at the shell prompt, and in other full screen editors. Why does this happen to me in vi?

Thanks
.