If you put a scope on your I2C Bus, you would have realised the waveform didn't meet the specification of Table 2-5 in the Datasheet. Some devices do not require PU Resistors, PICs do... the clue could also have been found in the PBP manual I2C commands where an example schematic is given connecting to an external EEPROM. 4K7 is a good value when connecting one or two slave devices with a relatively short track between them. You need to drop this value for more devices or greater track capacitance. Refer to Raph's pointer to the I2C Bus Spec.