I have been designing cavendish banana and mango ripening chambers for last 8 years. Once in a while bananas that are not 100 percent mature have turned hard and do not ripen even for 7 to 8 days. The ethylene concentration is fine and so also is the CO2 (below 0.25%) but the bananas will not ripen and become stone hard. Have you come across this phenomenon? (V.M.)
Immature-green bananas may fail to respond to ethylene and ripen even after exposure to 100 ppm ethylene for 7 days. You may want to work with the banana producers to harvest bananas after they have reached full three quarters to full circle in cross section (see banana maturity chart). You did not mention temperature, but it is the most important factor in ripening fruits and green bananas will fail to ripen if their temperature exceeds 35 C. If bananas at 35 to 40 C are received at the ripening facility (18-20 C) and placed very tight in the ripening room, it may take more than 7 days for the banana temperature to reach near 20 C. Such bananas should be cooled to 20 C before placing in the ripening room or at least they should be stacked with adequate spacing for good air circulation to facilitate their cooling.
-Adel A. Kader