LPI requirement on optical encoder
Simply put, the geometries of the devices are designed for a specific spacing between light and dark in order to maintain a true quadrature output.

If you don't match the device to the target lines per inch (and also spacing, and light/dark ratio's), the device cannot generate an accurate quadrature output.

Not getting any output could also depend on your setup.  Try to eliminate any ambient light hitting the sensor.  Or your gap from the sensor to the reflective strip may be incorrect.

If just the line density is incorrect, that could cause no output, but there is a 75% chance that you should see at least one output on (Quadrature signals only have one state out of 4 with a 0,0 output).