He lifted some more sanctions because his friend Kim has begun to rebuild the weapons test site in preparation for further tests. And if that test really begins, then Trump will have failed in his negotiation. And he can not have that … it would really hurt his ego.
So he has eliminated some sanctions in the hope that Kim will not continue to rebuild and use that weapons test site. It is called quid pro quo in politics. In simple words, Trump said: "I relaxed some sanctions, now you stop rebuilding that test site."