Question

That IS a good question. We know they can fix each other, because the female fixes the male after Carlos beat the microchips out of him with a bat. I guess the film's writers didn't consider that, or maybe they factored in that these machines have emotions and feelings like humans, so they would just assume one of their own was dead and beyond fixing. Much like we human beings can fix ourselves (by way of surgeons, doctors, etc.) but sometimes babies are stillborn and cannot be fixed. So to the machines, there was no hope for their defective little one. The help came from a higher being, or in this case, Harry. Either they wanted to have the subplot of him and the little guy, or show that these machines can fix human things, while humans can fix machine things?

The writers probably didn't think of it.

It's possible that while they can fix anything, the female had no idea that it was possible to fix themselves until she saw Harry fix her little one. Then when the father was nearly killed, she decided to try fixing him. Remember that unlike other broken things, that were repaired almost instantly, she took all night to fix the father. So it's possibly that the robots had no experience fixing themselves.

Maybe the robot has to have been 'alive' first before it can be revived...

Maybe Mama was too exhausted from having given birth to expend the necessary energy.