You are probably already saying what you need to be saying, some kids just take time to get it together.

Fast air
Small fast air
Cold air
Aim your air down
Smaller aperture

Sometimes I can coax a higher buzz by having them buzz on their mouthpiece and slide from low to high, then ask them to start on the high buzz they just ended on.

Double check and make sure they aren't doing anything weird with their embouchure- a lot of times my kids that struggle getting to pitch are pushing their lips outwards (duck face) or allowing the shiny inside part of their lips to come out around their corners. These will need to be addressed if they are going to make it. Some kids eventually fix it- some kids do not.

I will get some disagreement here, but sometimes I'll tell a kid to "buzz as high as you physically can" or "firm up your lips" and ignore what would be considered good pedagogy to get a kid over a hump like this. This sometimes forces some good changes and it is often easier for them to work towards making a good sound on the note once they have experienced a little bit of success.