Piskei HaRosh Sanhedrin 4:6
דאמוראים האחרונים פעמים חולקין על הראשונים ואדרבה אנו תופסין דברי האחרונים עיקר כיון שידעו סברת הראשונים וסברתם והכריעו בין אלו הסברות ועמדו על עיקרו של דבר
For the later amoraim at times disagreed with the earlier [amoraim], and on the contrary we grasp the words of the later ones as primary since they knew the reasoning of the earlier ones as well as their own reasoning, and they made a determination between the different reasonings and reached the main point of the matter.
According to this we follow the judges in our generation because we assume that they know what the previous judges ruled and still disagreed. We thus trust that they only disagreed because they evaluated the reasoning of the earlier judges as well as their own and decided that theirs was better. However, if the current judges are entirely unaware of the earlier material, then we might very well say that the current judges erred and would have ruled differently had they known what the earlier judges had said:
ואני אומר ודאי כל מי שטעה בפיסקי הגאונים ז"ל שלא שמע דבריהם וכשנאמר לו פסק הגאונים ישרו בעיניו טועה בדבר משנה הוא ולא מבעיא טועה בפיסקי הגאונים אלא אפילו חכמים שבכל דור ודור שאחריהם לאו קטלי קני באגמא הן ואם פסק הדין שלא כדבריהם וכששמע דבריהם ישרו בעיניו והודה שטעה טועה בדבר משנה הוא וחוזר
And I [the Rosh] say that certainly anyone who errs in the rulings of the gaonim of blessed memory, that he was unaware of their words and when made aware of the ruling of the gaonim it was proper in his eyes, is one who erred in an explicit Mishnah. And not only one who errs in the rulings of the gaonim, but even the sages in every generation after them "are not reed-cutters in the bog", and one who rules against them and when he finds out their words they are proper in his eyes and he admits that he erred, it is an error in an explicit Mishnah and is reversed.