Weird Home Brew Ginger Beer Problem?

Question

I prepared a batch of ginger beer a week and a half ago and put most of it into a flip top bottle. I had some of that same batch left over and put the remainder in a reused plastic bottle. The culture in the flip top is hardly fermented (low fizz and still

 sweet) while the plastic bottle is appropriately fermented (fizzy and a little more dry). These started out from the exact same starting liquid and have been stored in the same place next to each other. Is there something I should know about using the flip tops?

FYI. I just turned the flip top on it’s side to make sure there was a water tight seal and it’s not leaking. 

Answer 1

Not possibly, this is my first time using the bottles. I cleaned them out (a drop of soap in the bottle) and washed with an intense amount of water. I’m more worried that I didn’t choose the right material (glass?) but that seems to be the popular choice!

Answer 2

Any chance the reused bottles had some sugar in them?