Some words I’ve never seen before are not highlighted, so I try to highlight / select them so I can add a LingQ but they can’t even be highlight. And it only happens for certain words. I can pick a different word that is unhighlighted and select it with my cursor and I can then add a LingQ, but some are weird and I can’t. I try selecting the whole word, part of the word, everything. Why are some working and some not? And it seems all the words that I can’t select for some reason are ones I don’t know and ones I can select are ones I knew, even though none of them are highlighted in blue or yellow.
This appears to be a bug. Which language are you studying and which browser are you using?
I am learning Korean and I’ve had the same problem on Firefox and IE
Ah, I see. This is a known bug in Korean, and we should have a fix for it some time in the next week or so. Until then just do your best to ignore these words 
You’ll certainly come across them again, and any words that are marked as “Known” can be highlighted and LingQed if you forget them.
This should now be fixed. Remember to clear your cache otherwise you might experience some issues!