import folder function does not import all pdfs in folder

I have only recently started using EndNote and I am currently trying to import the pdf collection on my hard drive, which is divided bu topic into several folders. So I am using the Import folder function, which works well apart from when it leaves out random files. If I go back and try to import each file individually it’s all good, with all reference fields filled in correctly, but I really don’t want to have to do this for potentially hundreds of pdfs. 

Does this issue sound familiar to anyone and if yes, did you manage to solve it?

Thank you lots in advance!