the dosage that you have give to your wife is right, the possibility is that the pregnancy
test was positive after the medication is that she had already conceived before you gave the medication,as the levonorgestrol doesnt help if there is already a pregnancy present.
i would suggest get a ultrasound
done as sometimes urine pregnancy test can be a false positive,
the only solution to this is an abortion
which is controversial because in my country we dont do it on regularly basis because it is illegal and we only do it if the pregnancy is affecting the mother,
i would suggest that you and your wife go to see a gynecologist for further treatment
hope have been able to answer your query
take care wish you and your wife the best.