Why has my father's vision worsened after cataract surgery?

My father is 64 years old. He was suffering from high myopia in both eyes and was operated for cataract (phacoemulsification surgery) approximately two years ago. After surgery, he had some double vision problem. Doctor ruled out chances of squint and suggested that this problem would get resolved with time. Now, he is unable to read even newspaper under tube light and double vision problem also persists. Kindly suggest what should we do?