The Prime Minister has proposed his Tory colleague Lord Hill of Oareford, former leader of the House of Lords, to serve on Mr Juncker's new commission and is seeking an economic portfolio for him.
But the president, whose appointment was fiercely resisted by Mr Cameron, has indicated he is not happy with the gender balance of the candidates so far.
Just a handful of the 28 member states have nominated women and Mr Juncker said: "A commission without a significant number of women is neither legitimate nor credible. That is why I am continuing to insist with several heads of state and governments they send me a female candidate."
It comes as Labour accused the Conservative party of being a place where "women are seen but not heard".
Deputy Labour leader Harriet Harman criticised the Prime Minister's much vaunted recent reshuffle, designed to increase the number of women in the government. He is still far short of his stated aim to have one in three ministerial jobs held by a woman.