Two issues:

1. if you have upgraded xmonad past 0.11, you need more hooks for docks to work right. Consider switching from manual hook management to http://hackage.haskell.org/package/xmonad-contrib-0.13/docs/XMonad-Hooks-ManageDocks.html#v:docks (which will also protect you from future additions/changes).

2. For reasons currently unknown, ManageDocks is ignoring struts on windows that are not marked as EWMH dock windows. dzen doesn't mark itself as a dock by default; you must pass the `-dock' parameter to it.
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