I looked in my heart while the wild swans went over.
And what did I see I had not seen before?
Only a question less or a question more;
Nothing to match the flight of wild birds flying.
Tiresome heart, forever living and dying,
House without air, I leave you and lock your door.
The poet uses the repetition of ‘question’ to give it emphasis. The modifier ‘only’ is ironic; the question is the crucial aspect of the poem. 
 The line is rhythmic, juxtaposing the  antithetic  ‘less’ and ‘more’, perhaps to mimic the beating of swans' wings.
Wild Swans