8GB Macbook Pro Retina 2013 running MacOS 10.12.3, LR 6.0 256GB SSD only half full. All files on LaCie 2TB RAID 1 drive. 30K images in catalog that I'm trying to clean up. Mouse click to select will randomly do nothing. Other mouse clicks, like to a menu item, are fine. Select all or select flagged from menu seem to work fine with minimal delay. Once I do get a set of images selected, dragging the multiple images to an existing collection simply won't work at times, the target collection is never highlighted and nothing is copied/moved. If I randomly click around to other collections and LR objects and then come back to the original grid display, the click/drag of collection suddenly works.
'Where' you click in the grid of previews makes a difference.
When multiple images are selected-
Clicking on an image ‘picture’ will make that image the “master” image in the series.
Clicking on an image border or frame will deselect all other images.
Also, to drag and drop selected photos ( eg. from one folder to another) you must start the 'drag' by clicking on the image, and NOT the grey border of the preview. So the position of the cursor is all important.
thank you. clearly I missed this important detail in the documentation. Thanks for not just saying RTFM ;-)