JOEL - When people die their spirits enter the Spirit World and are classified according to the purity of their lives and their obedience to the will of the Lord while on earth. They may progress from one level to another as they learn gospel principles and live in accordance with them (see Bruce R. McConkie, Mormon Doctrine, p. 762).
Those who enter the Spirit Prison portion of the Spirit World are those who either were untaught and unrighteous, or were disobedient to the gospel while in mortal life (Alma 40:11-14; D&C 138:32).
Only those who would have truly embraced the Gospel of Christ if they were given the opportunity while on earth, will be the ones that would probably accept it after they die(D&C 137:7-8) and are taught by Christ and/or His disciples(1 Pet. 3:18-19, D&C 138:29-30). Those who would not have accepted it would probably not accept it in the next life either as Alma explains:

"Ye cannot say, when ye are brought to that awful crisis, that I will repent, that I will return to my God. Nay, ye cannot say this; for that same spirit which doth possess your bodies at the time that ye go out of this life, that same spirit will have power to possess your body in that eternal world." (Alma 34:34)

In other words, just because someone has now died and entered the after-life, conditions will be such that, even though they now know that there is in fact an after-life, it is not necessarily going to convince them of the necessity or truthfulness of the Gospel of Christ. When they die they will take that same spirit and personality with them that they possessed in mortal life, which will be a great influence on them in the next life. There are also many who have experienced a "spiritual death" while on earth due to a willful disobedience to God. This will also effect their ability to accept the truth. But just as there are those in this life that do make a change for the better, it is possible for those in the Spirit Prison to change as well when they hear the Gospel there.