Collocations for doubtful - adj.

be, feel, look, seem, sound
Eric was far from sure and Marion looked doubtful.

become | remain

extremely, very | a bit, a little, rather

about
She was rather doubtful about the wisdom of eating the two-day-old food.

be, look, seem | remain

extremely, highly, very
Even if we could go, which is highly doubtful, John wouldn''t be able to come with us.

increasingly | a bit, a little, rather, somewhat