Attachments do not attach

I am running EndNote X7 on an iMac. Occasionally I try to attach a pdf file and nothing happens. Literally, nothing. EndNote doesn’t even display the usual attach file dialog or give me an opportunity to navigate to the folder where the pdf is. Other pdf’s open OK.

I’m having exactly the same prob with X7.1 on Mac. Can someone help please? 

This problem disapperred after I quited the program and restarted again. Before I did this, I wrote to the Endnote support team and this is what they suggested, which might be very helpful if this prob persists: 

Check if this is a problem with a specific library or with all the libraries. You can do this by opening another one your libraries or from Applications\EndNote X7\Examples\Sample_Library_X7.enl 

If the issue does not occur with the sample library and if it is specific to a library, then they recommend to recover that library.  (remember to make a back up copy of your library before perform recovery).