- IE6 has a limitation on how long the query string can be so use POST instead of GET (for debugging purposes GET is good) in your production systems.
- IE6 is less forgiven than other browsers (which in my opinion is not exactly wrong) so pay attention to the correctness of the document. Always use a validator for your markup.
I still do not understand why I need to be writing about Internet Explorer version 6 in almost year 2011. It reminds me some dictatorships in the world: everybody knows it is evil, do not work fine, it is a waste of time, it is *not* efficient, and the list goes on. Still like some dictatorship for some reason millions of people still want to live oppressed apparently (read: use a FREE mozilla browser, FREE Chrome, FREE Opera).
Life is complicated and I cannot propose to ban IE6 as that will make me a dictator.
Developers are like doctors: no matter what the person is the doctor must save that life. We as developers must fix IE6 issues to save IE6 slaves.
Hopefully this will explain why that form of yours sometimes submits and sometimes simply it does not.
Interesting enough I have seen myself IE6 not submitting when receiving the form from the server however the very same form gets submitted if tested from a saved HTML file. But seriously I have no time to deal with it ...