Jacob 5 quotes Zenos’s parable of the olive trees, as Jacob explains in verse 1: Behold, my brethren, do ye not remember to have read the words of the prophet Zenos, which he spake unto the house of Israel”. But where in the scriptures does Zenos say that? The Bible Dictionary says that, The Book of Mormon makes reference to writings of Old Testament times and connection that are not found in the Bible, the Book of Mormon, or in any other known source. These writings are of … Zenos …” So we don’t have them. How did we loose them? The Spirit told Nephi that in 1 Nephi 13:23-29: “The book that thou beholdest … is a record like unto the engravings which are upon the plates of brass, save there are not so many … that great and abominable church, which is most abominable above all other churches; for behold, they have taken away from the gospel of the Lamb many parts which are plain and most precious; and also many covenants of the Lord have they taken away … And after these plain and precious things were taken away it goeth forth unto all the nations of the Gentiles”. So, we lost them in the many, many abridgements and translations. The Joseph Smith Translation can give us some of them back, but still, not all of them. Why? Probably because we’re not ready for them yet. But we will get them eventually.
https://www.churchofjesuschrist.org/study/scriptures/bofm/jacob/5?lang=eng&verse=#p