Nicene and Post-Nicene Fathers, Ser. II, Vol. XIV:
The Canons of the Synods of Sardica, Carthage, Constantinople, and Carthage Under St. Cyprian, Which Canons Were Received by the Council in Trullo and Ratified by II. Nice.: Canon CXXI

Canon CXXI.  (Greek cxxii.)

Of those who neglect the peoples belonging to them.

Item, it seemed good that whoever neglect to bring the places belonging to their see into Catholic unity should be admonished by the neighbouring diligent bishops, that they delay no longer to do this; but if within six months from the day of the convention they do nothing, let them pertain to him who can win them:  but with this proviso however, that if he to whom it seemed they naturally belonged can prove that this neglect was intentional and more efficacious in winning them than the greater apparent diligence of others; when the episcopal judges shall be convinced that this is the case, they shall restore the places to his see.  If the bishops between whom the cause lies are of different provinces, let the Primate in whose province the place is situated about which there is the dispute, appoint judges; but if by mutual consent they have chosen as judges the neighbouring bishops, let one or three be chosen:  so that if they choose three they may follow the sentence of all or of two.

Notes.

Ancient Epitome of Canon CXXI.

If any neglect what belongs to their jurisdiction, let them be admonished; and if they shall do nothing within a six month, let them be adjudged to him who can win them.  But if they have committed the neglect out of policy so as not to irritate the heretics, and this shall appear to have been the case, their sees shall be restored to them, by the judgment of the bishops either appointed or elected.

This is Canon xiij. of Carthage, a.d. 418.