Made ginger beer, but couldn't get it to carbonate

So I tried my hand at making some ginger beer a few weeks ago, following this recipe. I decreased the total volume to make about 6 cups rather than the full 2 gallons (I only have 2 flip top bottles at the moment). My ginger bug had small bubbles when stirred it, and I waited for the mix to cool fully before adding the bug. I poured it into the flip top bottles and burped them every day for almost 5 days without seeing any fermentation. I ended up drinking it mixed with some sprite to add some carbonation, and it was lovely (not too sweet or spicy) but obviously the goal was for it to carbonate on its own.

Any obvious points of failure here or things to try? My only thought is to not burp them daily and instead use a plastic bottle as a tester for pressure.