The reason Nicodemus had not been born again was his own fault for man has a role to play in his own new birth.
At that time in Jn 3, John’s water baptism was in effect (Jn 3:23), John’s baptism would remit sins (Mk 1:4) yet Pharisees as Nicodemus rejected John’s baptism (Luke 7:30).
John’s baptism has been replace by Christ’s water baptism of the great commission (Mt 28:19,20 > Acts 2:38). Those that submit to Christ’s baptism (born of water) have all sins remitted then can walk in newness of life (Rom 6:4).
Those who submit to Christ’s baptism are baptized into Christ therefore are clothed in Christ (Gal 3:27). Having put on Christ in baptism, one is covered by Christ’s perfect righteousness and therefore seen by God as perfectly righteousness. Christ’s righteousness is not transferred to the sinner while the sinner sits idle. But the sinner obeys the will of God in submitting to baptism and then is born again having put on Christ.
I have never been a fan of Calvinistic dogma. Of course each of us must answer for the behaviors we have done / do in this life, and the Bible tells us that by not choosing The Grace of God in Christ we have condemned ourselves for not believing God. I do not believe any Baptism remits sin, but the submitting and following through is an act of the will and when the will of one repents of sin, that is when the remitting happens and the baptism is an outward sign of same. When Jesus submitted to John’s Baptism there was not sin to remit. Jesus did it to fulfill all righteousness.