You put two of the same sensor on an I2C bus and one doesn't respond. Why?

Almost certainly an address conflict: I2C devices are selected by address, not a chip-select line, and two identical chips ship with the same fixed 7-bit address, so they collide on the bus, both try to respond to the same address, corrupting the transaction, and effectively you can't talk to them independently. The fixes depend on the chip: many I2C devices have one to three address-select pins you tie high or low to choose among a small set of addresses, so you give each sensor a different address; if the chip has no selectable address (or you need more copies than it allows), you put them on separate I2C buses, or use an I2C multiplexer/switch (like a TCA9548A-style part) that gates each device onto the shared master bus one at a time. The general lesson is that I2C's two-wire elegance comes with a finite address space and no per-device select, so device-address planning is part of the design, unlike SPI, where you'd just add another chip-select line. Before blaming hardware, confirm the two devices' configured addresses actually differ.