Some members are getting charged GST and others aren't?

Heya,

We recently launched a new members area with subscriptions. 

I've just noticed some of our members are being charged 10% GST successfully, and others aren't.

I've had a look around to try and work out why but cannot for the life of me find the issue. I've posted a screenshot below. The monthly subscription cost is $49. You can see where they were charged the 10% and where they weren't. This is all from the same week. We only have 1 members area. 

What am I missing?