Inconsistent journal abbreviations in biblio using Refman 11

Hi, im using refman 11 and have been trying to use the basic index medicus abbreviations in all the ref’s in my bibliography. Ive set the periodical synonym as “standard abbreviation” in output style. However, in my biblio, a journal will be abbreviated properly in one reference but inthe very next reference, it will be the full name. I need the journal name abbreviated in every reference. Suggestions appreciated.