Pull-up selection depends on bus capacitance, required rise time and participant sink capability. Added devices often add capacitance, but simply lowering resistance can violate low-level current or VOL limits; calculate and measure both bounds.
Firmware must match the target address (often influenced by strap pins), supported bus mode and register map. Two targets responding to the same address can both acknowledge; later read data or command interpretation may conflict rather than producing a reliable NACK.
Verify pull-ups on board schematic — missing pull-ups = no communication.
Reset sequence: device must be out of reset before I2C access.
Shared segment: ensure simultaneously enabled targets do not respond to the same address unless a mux, enable or other selection scheme isolates them.