I have this problem sometimes as well. Even though it may look like you have enough space to install it android wont let you because it likes to keep some space free for system updates. In order for me to install anything I usually needs to male sure I have at less 2gb of space for any thing under 1gb in size. If that doesn't work try resetting your phone or tablet I found that this works some times as well. If you're trying to update it you may need to uninstall it and in install the new one. Some of the VNs on I got on my phone refuse to update and I end up haveing to uninsall it to get the new stuff.