Try turning off the GPU acceleration, go to Preferences, click on the Performance tab, then uncheck the box.
Also, are you doing a lot of spot healing and/or brushing on each photo?
What CPU do you have? How large are your photos in megapixels? (Do not tell me megabytes)
What type of keyboard are you using? I also have had this problem and I write it off to my wireless keyboard that is solar powered. At times my keyboard does not respond to key presses. Rare but it does happen every now and then.
i use wireless keyboard and mouse...maybe i should unplug and restarted..i will try..
intel core i7 -4720HQ CPU2.60Ghz 2.59Ghz,16 gb RAM,64 BIT PROCESSOR...usually i use 20mb raw file with LR6 to adjust the file and convert to tiff file..thanks.