CVS and diff-window with conflicts

Why doesn't Idea alway pop up with a merge conflict window. Sometimes it just shows the conflicts in the source.

Scenario: Branch from a main-trunc, - makes some fixes on the branch, - committing them (they'll cause conflicts).

Jump back to main-trunc, - makes some changes, - committing them. Merge the branch in to the main-trunc. I get som conflicts, - but no diff-window with merge-conflicts.

Why?

Rgds.

Geir A.