The external device is not able to pull down the lines(s) lower than the reference voltage Uref.
Serial resistances on the bus are too high
The pull-up resistors are too low
Figure 1 shows a simplified schematic of the I2C bus (only SCL shown). Assuming Tracii XL 2.0 pulls down the line as an external device, the serial resistor Rs and the pull-up resistor Rp work as a voltage divider. A high value of Rs results in a high remaining voltage U2. If this voltage U2 does not fall below Uref, Optii is not able to detect the active device.For proper operation, a serial resistance Rs of 0 Ohm would be the best. If a decrease of the serial resistance is impossible, increasing the pull-up resistor Rp will also enhance the situation, but may limit the maximum transfer speed.