Although they (the Yamadutas) had gone to arrest Ajamila, following Yamaraja's order, they found themselves unsuccessful because of the order of some higher authority. Therefore they were unsure of whether there were many authorities or only one

Expressions researched:
"Although they had gone to arrest Ajāmila, following Yamarāja's order, they found themselves unsuccessful because of the order of some higher authority. Therefore they were unsure of whether there were many authorities or only one"

Srimad-Bhagavatam

SB Canto 6

Because the Yamadūtas had been unsuccessful in carrying out the order of Yamarāja, they doubted whether Yamarāja actually had the power to punish the sinful. Although they had gone to arrest Ajāmila, following Yamarāja's order, they found themselves unsuccessful because of the order of some higher authority. Therefore they were unsure of whether there were many authorities or only one.

SB 6.3.5, Translation and Purport

If in this universe there are many rulers and justices who disagree about punishment and reward, their contradictory actions will neutralize each other, and no one will be punished or rewarded. Otherwise, if their contradictory acts fail to neutralize each other, everyone will have to be both punished and rewarded.

Because the Yamadūtas had been unsuccessful in carrying out the order of Yamarāja, they doubted whether Yamarāja actually had the power to punish the sinful. Although they had gone to arrest Ajāmila, following Yamarāja's order, they found themselves unsuccessful because of the order of some higher authority. Therefore they were unsure of whether there were many authorities or only one. If there were many authorities who gave different judgments, which could be contradictory, a person might be wrongly punished or wrongly rewarded, or he might be neither punished nor rewarded. According to our experience in the material world, a person punished in one court may appeal to another. Thus the same man may be either punished or rewarded according to different judgments. However, in the law of nature or the court of the Supreme Personality of Godhead there cannot be such contradictory judgments. The judges and their judgments must be perfect and free from contradictions. Actually the position of Yamarāja was very awkward in the case of Ajāmila because the Yamadūtas were right in attempting to arrest Ajāmila, but the Viṣṇudūtas had baffled them. Although Yamarāja, under these circumstances, was accused by both the Viṣṇudūtas and the Yamadūtas, he is perfect in administering justice because he is empowered by the Supreme Personality of Godhead. Therefore he will explain what his real position is and how everyone is controlled by the supreme controller, the Personality of Godhead.