Not currently being in His flock could be the cause of them not trusting Him – the grammar allows this meaning.
This would run along with a generally uncontroversial theory that until whenever the Holy Spirit calls someone, they’re unable to come to trust God – along with other generally uncontroversial theories such as, so long as someone is rebelling against being in the flock they are unable to come to trust God. (These ideas should still apply regardless of which Christology and Pneumatology is true, btw.)
How those ideas get applied can be controversial, such as a Calv belief that God never intended, much less acted, for some persons to be in His flock. I’m arguing elsewhere according to context that they’re not in His flock by being rebel members of the flock, not because He’s excluding them from His flock. Similarly there are two ways someone can be your king: you can accept and follow him as your king, and he can be in royal authority over you. These aren’t mutually exclusive, and a king can be in proper authority over someone who rebels against that proper authority. Consequently in one important sense he is not their king, but in another (I would argue more) important sense he is still their rightful king.
This really shouldn’t be controversial either – even Calvs acknowledge and agree with the principle – but then Calvs turn around and force a controversial schism in the idea in order to stick with their explanation for how the Gospel assurance of God’s victorious salvation can be true and yet some sinners are permanently lost after all: they aren’t in the flock because God (including the Son, in trinitarian Christology) never intended for them to be in the flock at all, therefore never empowers them to be in the flock much less ever leads them to be in the flock.
But the grammar of that one place does not necessarily require this idea; and the immediate and local contexts exclude that idea (I’m arguing along the way.)