Publications data obtained from different sources can often be difficult to reconcile. You may, for example, want to link citation numbers from InCites with imapct data from Altmetric, but these two sources will use different identifiers for papers.
Usually, the best way forward is to use the DOI to link the two datasets together. While this is not guaranteed to be a unique identifier, it will work for 99% of cases. However, of course, it will not work if your source data does not contain DOIs.
You may encounter problems in particular with books - these are not always indexed on the same "level". For example, Scopus might index a single "book" item with a single author, plus another twenty seperate "chapter" items with different authors. Web of Science, meanwhile, might index it as twenty "chapter" items, all with different authors, but the introduction chapter is given any citations for the work as a whole. And finally, Altmetric might only index a single record for the whole book.
If you are trying to link data to records from PubMed, then InCites data exports will always contain a PubMed ID where known.
Linking data can often be challenging - please feel free to get in touch if you would like assistance.