Measure.
If a 500 Ohm resistor can't pull down the bus it could be
- that you have connected another pin to the bus, i.e. you are messing with the wrong pin
- that something else is pulling the bus up
Therefore measure what is really going on on the bus, e.g. how much current is going through your pull-down resistor.
And if you are willing to risk some magic smoke, use a stiff wire to pull the pin to ground. The component that starts to get hot or releases the magic smoke is likely the one trying to pull the bus up.