They never forget they exist, they just don't use them. The cast was already huge by the time StrikerS started and it added (literally) dozens more.
Arf doesn't appear often because she wasn't popular. Her role was already dramatically reduced in A's, remember. She appears several times in StrikerS and the manga fully established why she opted not to be at Fate's side anymore (she feels Fate doesn't need her as a guardian anymore).