In some cases the publisher may regard the abstract as copyrighted material that they
do not wish to disclose. In today's world where some parts of publishing is dominated by commercial publishers, this is to be expected. In other cases the abstract may simply be too complicated to reformat in JATS format for reporting to crossref. We face that problem since we allow
TeX mathematics (even
display mathematics) and itemized lists in abstracts, but JATS has poor support for these in an abstract. The JATS standard for abstracts is too limited:
https://jats.nlm.nih.gov/publishing/tag-library/1.4/element/abstract.htmlYou probably shouldn't expect all abstracts to appear in openalex until the publishing world changes more things.