i2c bus isolation using Si8400
I think if you haven't got bypass caps present, then this is very likely to be the problem (especially given the RF isolation)
The datasheet recommends 1uF ceramic caps be placed on both sets of supply pins. Put them as close as possible to the pins.  

Also, make sure you have the pullup resistors present on both sides.   

If you have an oscilloscope, you should be able to test the lines easily (e.g. send repeated signal and see if what's going in one side is coming out okay the other)