What playback devices have you tried, where Boost with regards to HD picture quality isn't supported on PC's or smartphone / tablet, 1st or 2nd generation Chromecast.
Also to run Boost in HD picture quality NOW recommends 12Mbps internet speed.
Under My Account on your NOW playback devices on your television, does it show the Boost Membership active.
Under the Sports TV Guide does it show HD at the end of the Sky Sports channel names ?
if you have certain NOW playback devices, pressing the info button or ok button will show the streaming quality (give it about 25 seconds to ramp up and a figure of around 7000 means HD 1080p picture quality.
Also a day sports membership will work on Boost.
Can you provide more information about the actual NOW device you are using ?
Is it a NOW box or NOW Stick or a LG Smart TV or a Samsung Smart TV with the native inbuilt App or another device ?
Have you given the sports live stream about 25 seconds to ramp up to HD 1080p ?
You could try a reboot of your playback device and Router.
If you couldn't watch your day sports membership, i would normally suggest try watching it on a smartphone / tablet or PC first to trigger the day membership (but it sounds like your day sports membership is already working).
Maybe try another log out and login again on the NOW App under My Account on your television.