Collections - Problems to add some photos into a Collection

[#1]

Hi all. It happens to me that LrCC doesn't allow me to add some photos into collections. I take the pic and try to drag it, nut nothing happen. With the same folder of library, I can take and drag some files but it dosen't work with some other file of the same folder. 

I am getting crazy!

[#2 · Accepted solution]

Make sure you drag the image itself, not the grey border.