It could be that the file is being uploaded and it's in use technically, and it can't be added to the document until it's not in use any more.
Yes, I thought of that but other software recognized the file right away. The recent example is a 39.3MB file took 45 min to be recognized by INDD in a folder where Mac OS saw it right away. So I just experimented - I reserved the file down to 11MB and tried to place. INDD still doesn't recognize the file after 5 minutes. I then tried to open the new, smaller file from within PS and see that it also doesn't recognize the file (after 5 min). However, I can double-click the file and it will open immediately in PS.
This tells me there's something funky with CC. I was not having this problem with CS6 running the same Dropbox system.
It's now been 7 min and INDD still doesn't see the new, smaller file when using the place command.
I use DB constantly and have not noticed this, but will keep an eye out.