A stock objection to reprobation is that reprobation is unjust. Typically, no actual argument is given for the injustice of reprobation. Rather, a critic defines or describes his understanding of reprobation, then declares it to be unjust. So there's really nothing to respond to.
That said, let's consider a comparison. Suppose, on his daily walk, my dad sees a red Ferrari in a parking lot with the keys in the ignition. He always wanted a red Ferrari, so he seizes the moment and steals the car. He then takes it to a chop shop to change the license plates and vehicle registration. A little money under the table goes far.
In the course of time he wills the car to me. When he dies I inherit a classic Ferrari.
But one day the son of the original owner (who has since passed away) spots the car in my driveway. He pops the hood and confirms the serial number matches his late father's car. He then has the police repossess the car.
It is unjust that I lost the Ferrari? No. It was stolen property. I had no claim on it in the first place. Because it didn't belong to my dad, he had no right to give it to me.
There is, though, a sense in which it's arbitrary for the other son to claim the car. He didn't pay for the car. His father did. It was simply a gift. Something he inherited. He didn't buy it. He didn't earn it.
The deprivation of something we were never entitled to is no injustice. To be deprived of election is not unjust. Conversely, the elect did nothing to merit election.