I abandoned the Covenant: Way of White. How can i get back to it?

As the title mentioned above.

At the first time i joined Covenant: Way of White and i abandoned Way of White because u want to enter another Covenant.

At some time i think i need Way of White better than another covenant, so im going back to the fat guy at firelink shrine to re-enter the Way of White. When i talked to him, i can't re-enter Way of White, there's no longer any options to purchase miracles or to enter the Covenant: Way of White.

Any ideas? Thx