Transitions in Premiere Pro CS6

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Joined: Sep 3 2002

For some unknown reason (to me) Premiere Pro GS6 transitions are no longer working.

Transitions can be selected and placed before, after or centre of cuts on a timeline.

Details of a transition can also be displayed when selected (clicked).

However, when the timeline is played no transition is displayed although it is visible on/between cuts.

Is it possible that transitions can be "turned on/off"? Any remedy please?