Lightroom 5: Does not synchronize nor import files to its library

When I'm trying to import and synchronize a folder to Lightroom 5 I can't. Lightroom goes through the import procedure, i.e tells me how many pictures there are to be imported etc. and when I press the synchronize button everything seems to be alright... But, then a sign comes up saying there are no pictures or videos to be imported. But there are. I can see the folder I want to import on my external hard-drive, I can see it if I use Bridge, but if I use Lightroom 5 I can't.