Problems shifting new china kart

[clueless]

Hey all, I bought my kid a 110cc go kart. It has reverse, nuetral, 1st, 2nd, 3rd (no clutch). The kart is about 10 days old and under warranty but I figured I ask. Today when he went to drive it, it wasn't shifting gears properly. Ex; from nuetral to reverse it wouldn't go but if I roll the kart just a little bit it clicks in. Is there something that can be adjusted? I tried to tighten and loosen the shift cables but it did nothing. Tried to tighten the chain a little also but that didn't do anything either. The thing worked perfectly 2 days ago when he last drove it. Any help is appreciated. Thanks.