Sequencer Operators (bpy.ops.sequencer) do not all work!?

Several sequence operators (bpy.ops.sequencer) do not work.
For example:

bpy.ops.sequencer.paste()
bpy.ops.sequencer.duplicate()
bpy.ops.sequnecer.duplicate_move()
bpy.ops.sequencer.effect_strip_add()

I get a ‘context is incorrect’ message.

Any clue as to what is going on?