Update references can't find missing years


I have a large library  (1400 references) from a systematic search, the data has been exported and imported (Rayyan, Endnote Import File) and I have a lot of missing years showing blank (500+ records). Not sure if they were lost in process or were missing all along. 

I used ‘Find Reference Updates’ for blank fields to fill in the gaps. EndNote says the process is complete and all references up to date but the missing years have not been populated, which seems odd as most are  journal papers with DOIs and the years are clearly visible in the citation downloads for the sample I’ve manually checked so far. 

Any suggests

(EndNote X9.3.3 University licence on Windows 10 laptop at home)