Won't install ALL apps, or just some apps? You could try archiving and unarchiving first. Do you have an SD card installed? If not, does the app require an SD card? (Like evernote?)
And read this thread about uninstalling, restarting, etc.
Oh, and what device are you using? Color or Tablet? If a Color, when purchased and what's the partition for apps? (Originally when sold, very small partition. You had to take it into store to get repartitioned for more apps.) And if Tablet, what is the size? 16G or 8G? They have different sizes for B&N content too.