MCP23017 Interrupt Feature Not WORKING

I'm interfacing the atmega2560 to an MCP23017 port expander. I've set the device up to generate an interrupt if the value detected on port A changes from the DEFVAL register which is all high pins (0xFF). The problem is, the device seems to be not generating any interrupt when I ground a pin. This issue has been driving me crazy for a while, I have had no luck fixing this. I'm sure its just a stupid mistake or something I overlooked but i cant think of anything. All help is very appreciated.

Below is a portion of my code that is initializing the device. Basically they are just function calls that take the device addr, register, and value as the parameters. Am i initializing the device improperly or something?

expanderWrite(I2C_TOGS, IODIRB, 0x00);

 expanderWrite(I2C_TOGS, GPIOB, 0x00);

 expanderWrite(I2C_TOGS, IOPOLA, 0x00);

 expanderWrite(I2C_TOGS, DEFVALA, 0xFF);

 expanderWrite(I2C_TOGS, INTCONA, 0xFF);

 expanderWrite(I2C_TOGS, GPPUA, 0xFF);

 expanderWrite(I2C_TOGS, GPINTENA, 0xFF);