1) not adding enough scent. Check the recommended usage rate, I add 1/2 ounce fragrance or essential per pound of soap
2) essential oils are notorious for fading, especially citrus. Five fold orange sticks better than orange EO and I add Listsea Cubica to help the scent stick. I was told clay helped hold the scents but I have not seen that for myself.
3) fragrance oils - stick with a vendor that has proven to have long lasting scents - I love Peak and Crafters Choice
4) Check the fragrance threads, a lot of times fast fading scents are listed by those who have gone before
5) keep experimenting to see what works for you and good luck!