When using Look up feature in apple books, it shows “Nothing to look up” and while using Search web it takes me to safari and no results are found there also.

I've just begun researching unexpected behavior of the "Look up" popup menu int Apple Books myself. My query was based on UK-local names of birds in Bobby Burns poems (within an anthology of his poems that I purchased a few days ago. Selecting some text and then "Look Up" from the drop down menu command does NOTHING, but search initiated by typing that text into the search box brought up my the macOS-wide <Command-Space> "Spotlight Search" menu typically yields the desired information. I considered the possibility that the function of the "Look Up" menu command might be to find other occurrences of the same text in the same "book" (document) in the Books app, but that's not the case, because a few of my ATTEMPTED lookups were for words that did have multiple instances.

One key question I think is whether this happens everywhere or in just some pages or books/documents. I have run into certain pages in books where it does not work despite working on all the other pages. It's possible that the problem lies with the text formatting rather than the lookup function.

Does this happen in all books? Perhaps what you are trying to look up there is not text but graphics.