In the process history window, many of our processes do not display changes to t

Are you passing any of those process variables into subprocesses by reference? If so, the value change will be seen in the process history window of the subprocess, rather than the parent process.

No bishnu - they are passed explicitly. I definitely can't see the history for hidden variables, and I'm pretty sure I can't see updates to parameters either.

Alastair,
Changes to Hidden variables are not available in the History window.
See: forum.appian.com/.../Configuring_Process_Variables