Starter Help

I started my starter about 3 weeks ago and it smells good and it was doubling for a couple days but now it only rises a bit. I did the float test and half of it floated and the other half sunk immediately. It’s probably something with how I’m feeding it, I feed it two spoonfuls of flour and a spoonful of water everyday, but I’m just not sure how I should be feeding it. It also might be a little cold in my house since it’s been a little cold out recently. I’m just not sure what I’m doing wrong and would love advice because I want to bake with it as soon as I can!