A6 and A7 are analog input only on a Nano, those do not exist on an UNO.
The bootloader does not matter, the I2C bus will work the same with either bootloader, or no bootloader at all.
What voltage is the power supply for the I2C devices?
How are the I2C devices connected to the Nano? Do you have a Nano without headers, and if so are your connections soldered?