I have short hair and it seem that I need a relaxer every two weeks because the back of my hair seem to get nappy quicker because it is cut the lowest what can I do.

Two Suggestions:

1. Straight all your hair once a month. Straighten the back part you're having issues with every 2 weeks.

2. You could FIRST apply the solution to your back part and then the rest of your hair. That way the solution will be on the back part the longest and that should help it relax longer.