Altmetric is able to recognise a variety of unique identifiers from ORCID profiles, including DOIs, PubMed IDs, Handle.net identifiers, arXiV IDs etc. However, you should ensure that the research outputs that are manually entered into your ORCID profile have no typos in their identifiers and that all the identifiers are stored in the correct fields.
When manually adding works into your ORCID profile, you should always first enter identifiers into the "identifier value field" (this is the part that Altmetric looks at!) and not the "identifier URL field". Usually, the identifier URL field will automatically be populated once you have entered the identifier value field.