Interrupt Pin 20/21 Not Responding

Hello,

I am attempting to using five interrupt pins on an Arduino Mega. I have built a shield and have five IR photo-transistors attached to 5 interrupt pins. For each IR photo-transistor, there is a 10k resistor in series. For an unknown reason, pin 20 requires a 5k resistor while the pins 19, 18, 3, and 2 work well with 10k resistors. Before I post my code and schematic, I was wondering if there is a physical reason for this. I could solder in a 5k resistor, but I'd like to know why only pin 20 requires this change. Pin 21 has the same issues.

Best, Justin

Pins 20/21 double up as SDA/SCL pins. Do you have an I2C device connected?

No. There is a phototransistor connected to the pin. I have a IR diode connected on a separate pin to create an IR beam. When this beam is broken, the interrupt pin is activated.