To be all-knowing does not mean one is all-powerful, for one can know perfectly well how to do a task and be unable to do it. Anyone reading this probably knows how to twist the lid off a jar (you grab it and turn left, it’s not really difficult to know), but I bet most of us have encountered at least one jar we could not open. Sometimes, turning to tools works, and sometimes we call someone with more arm strength. Similarly, we know how to pick something up and hold it over our heads, but we don’t have the strength to do so with certain objects.
One might argue that being all-knowing would include knowing how to change oneself to be more powerful, but there’s no guarantee that an all-knowing being would be powerful enough to perform whatever necessary to become more powerful. Of course, such a being, as far as I know, has not yet been proven, so this is purely hypothetical.
However, does being all-powerful, as sometimes argued, include being all-knowing?
There are instances where one is technically capable of a task but does not know how to do it. For instance, I’m sure I was capable of solving algebra problems earlier than I learned them.
One could argue that being all-powerful means one is capable of making oneself know anything, and I find that argument pretty effective. It would not mean that an all-powerful being is all-knowing by definition, but such a being would be all-knowing in effect, for he/she/it could gain knowledge at will. The flip side is that he/she/it could lose knowledge at will.
Writing an explicitly all-powerful character would probably not be very easy. The interest of a story, typically, is in the struggle of a character to meet some goal. An all-powerful character would not need to struggle at all, obviously. He/she/it could just solve whatever problems. You’d have to artificially place limits to make the story interesting, and then the character just wouldn’t be all-powerful anymore.
An all-knowing character, though, or at least an almost-all-knowing character… that might be interesting.