Bṛhatī ad 6.1.1 says na pravṛttapravartane prayogaḥ āmantraṇādiṣu vyabhicārāt, literally: “[Exhortative endings] are not used to promote people who are already active, because of the deviant case (vyabhicāra) of invitations”. 

In fact, Prabhākara appears to believe that commands are always imparted on someone who is not yet active and who becomes active upon hearing them. The addressee of a command  desires something already and recognises themselves as the addressee through such desires (see Freschi 2012), but they are not active until they become enjoined.

Now, if Prabhākara means that active people are never promoted to act, why are āmantraṇas a good example? Orders (ājñā) are not used as a standard example because their being a clear case of promoting someone not already active had been attacked by the opponent in the previous line.

As for āmantraṇa, the situation with āmantraṇa is ambiguous, but Śālikanātha takes it as connected to a command uttered among peers (thus, it is not a clear case of a command uttered for people who are not already active). Moreover, what is the role of the vyabhicāra argument? How can a vyabhicāra argument be used to convince someone that something is *always* the case? A vyabhicāra can be used to show that it is incorrect to claim that “All As are B”, since there is at least one A that is not a B, but it can’t be used to corroborate “All As are B” by showing a further case of an A that is indeed B. In other words, why speaking of “vyabhicāra” and not of “yathāmantraṇeṣu” or the like if this was what he meant? The only possible explanation seems to be to think of āmantraṇādiṣu vyabhicārāt as “because the case of invitations, etc., deviates from the [opponent’s] claim”.  

Still, if it is impossible to command someone who is already inclined to act, how can Prabhākara make sense of permissions? When we are already about to light a cigarette and look around and ask whether it is OK and someone tells us “No problem, go ahead!”, isn’t the “go ahead” a command directed to someone who is already active? Could Prabhākara perhaps say that a command cannot enjoin someone who is already active because it would miss the apūrva element?