Actually, on second thought, your problem might rather be that the subprocess' output buffer is filling up... so it stops processing anything (i.e. hangs/waits) before you get a chance to read/emtpy the buffer (as you're doing the reading after the writing).  Just another hypothesis, though :)

How much output would you expect when things don't work?

As to diagnosing what's going on, strace would probably be a good tool.