ISBN identifiers filled in says give me metadata
for this ISBN
Identifiers blank says try and find by title or title-Author
ISBN does (MUST) change with edition (includes any publishing house change)
Not found problems can be traced to widely differing values.
Extra stuff in title field
Spelling errors
inconsistent Author name usage (their (the lookup source) or Publishers end)
[How many ways can you represent your name? That is how many variations are in the Metadata Sources
or on book covers ].
Fixing them in your(Library) DB before getting other metadata can lead to lookup failures