How to increase carbonation?

[BellGinAle]

I just transferred my cider from 2 liters to 25 oz. bottles, and I tried a taste. It was fizzy, but not like soda or beer, seemed kinda half-flat. Is there some way that I can help increase carbonation better?

[ezcaps_phpbb3_import2]

When you say you put them into 25 ounce bottles, what do you mean exactly?

[BellGinAle]

I am using some of my empty mr. beer plastic bottles, they are 740 ml each. I let my cider ferment for over 2 weeks, it was slow, but when I took a taste, it was too sweet, not very fizzy, and gave me a little stomachache. It was pasteurized though, and I put 2 cups of sugar in each 2 liter, so maybe I stopped it too early, or added too much sugar. I wondered, is it possible for me to fix it by fermenting it again, if I take out of fridge a while, add more yeast ?