It could be any number of things. Especially if there is a lot running in the background when you are trying to download. Some apps take up an awful lot of resources. The app may also require more space to install than it says is its 'size' to download it. Then you also have the factor of are you looking at how much is left via what you 'should' have on the device or what it actually says is left, when you check in the settings? You will get different answers depending on which info you are basing your numbers on.
What I would suggest is rebooting the tablet, don't run anything, and then try the download again.
OH and if you are including how much data you have available on an SD card in your numbers, I wouldn't. Not unless that card has been converted to be "Internal" storage. If it hasn't, then it will not help with the download.