Dimensions relies on the unpaywall data set to flag articles as Open Access and for the full text button. If a publication is not included there, then a full text button is not shown. 

Definition of Open Access filters

All OA
A free version of the publication is available (publication is either Gold, Hybrid, Bronze or Green OA)
Publication is published in a fully open access journal (this includes all publications with a Gold OA status in Unpaywall and those on our own fully OA list of journals)
Publication freely available under an open licence in a paid-access journal (Unpaywall OA status = Hybrid)
Publication freely available on publisher's website, but without an open licence (Unpaywall OA status = Bronze)
Publication freely available in an OA repository (Unpaywall OA status = Green, or publication type in Dimensions = Preprint)
ClosedNo freely available version has been identified (publication is neither Gold, Hybrid, Bronze nor Green OA)