I'm working as part of a team of 10 on an agile project, this is our first time using GitHub. We write our code, create a pull request and then start on our next task. 95% of these pull requests are being reviewed by our scrummaster/manager who tests whether everything is working but generally doesn't look at the code. Another issue is that he's a bit overloaded so PRs can take several days to be seen and if the code is important I'll sometimes just merge immediately after testing it myself.
I'm definitely guilty of not bothering to check other people's pull requests as it's quite disruptive to immediately stop what I'm working on to look at code that has nothing to do with what you're working on. I'd also get considerably less done than everyone else if I started to accept PRs from all corners.
Who should accept responsibility for reviewing someone's pull requests and how can we ensure this responsibility is not laid upon the same people all/most of the time?