Tyler Paul

Clean the rims and rough up the pads with some sandpaper.
References :

At 2 days old they may not be fully bedded in yet, so they may get better over the next few rides.

You could check to see that the brake blocks are flat against the wheel rims, and that the cables aren’t kinked.

If you can’t get them to work better, take it back to the shop you got it from, and ask them to set it up for you. They should have done that before you left the shop.
References :

every situation can be a bit different, but what should work it tightening the brake line. go down to the brake. with an Allen key, loosen the screw where the line meets the brake mechanism. pull the line tighter, but not to tight (you need to make a judgment call. this is a good place for some trial and error). screw the line back into place. test it out. i probably made it sound complicated, but it should only take a few minutes.
References :
i have fixed my own brakes several times because I have a shitty target bike that constantly needs work.