Import .pdf

I have been sending .pdf files to be imported, and have not had any problems until now. Several days ago, I was sending files, and they were all successfully imported, and marked as ready to download after 3 or 4 minutes each. Then suddenly, one that I sent has been "queued" for several days. It won't allow me to send another file, until this one starts processing, so essentially the system is locked up. As far as I can know, the file is no more complicated, or different in any way, to the ones I've sent before, successfully. What should I do?