On round one, K'zard the wizard casts Flaming Sphere.
On round two, K'zard doesn't sustain the spell.
Can K'zard sustain the spell on round three?
Role-playing Games Stack Exchange is a question and answer site for gamemasters and players of tabletop, paper-and-pencil role-playing games. It only takes a minute to sign up.Sign up to join this community
About sustaining spells:
If the spell’s duration is “sustained,” it lasts until the end of your next turn unless you use a Sustain a Spell action on that turn to extend the duration of that spell.
I think this is pretty self explanatory. In your example:
1st round - K'zard the wizard casts Flaming Sphere.
2nd round - K'zard the wizard has to choose whether to use the Sustain a Spell action or not. If he doesn't use the action the spell ends when this turn ends. If he does use the action, the spell continues until the next round.
3rd round - If K'zard the wizard chose not to sustain the spell on the previous round, there's no spell anymore, he can't try to sustain something that isn't there. If he did sustain it, he can choose whether to use the Sustain a Spell action again or not. Repeat this process on following rounds.