Why won't the XL cartridge work?

[1_Question]

At work we have an HP OfficeJet Pro 9015e that just ran out of black ink. It was my turn to buy printer ink / recycle our used cartridges and I was very careful to select the right cartridges (since in the past my boss had bought the wrong sizes, but that's besides the point). The empty cartridges that I was recycling were 962XL for black and 962 for the three color cartridges. That is exactly what I bought, I went back to work, and for some reason the new 962XL cartridge wouldn't work.

My coworkers and I tried all sorts of troubleshooting tips that we found on the Internet, such as cleaning the connecting points on the cartridge (we didn't clean the printer's connection points, as we didn't have the resources to safely clean them), unplugging and plugging back the printer, reinserting the old cartridge for 10 minutes before putting the new one back in... Nothing. We kept getting errors saying that the cartridge was missing, damaged, or couldn't be read.

By the time it was time to close, my coworker pointed out that one weird thing that she noticed; the old black cartridge and the new one had a different code on the top right corner. That's when I finally noticed that the black cartridge that we had just finished the ink for was the standard 962. Not XL. I panicked, worried that I had bought the wrong cartridges, but the Internet is also saying that HP printers normally don't differentiate between standard and XL cartridges because the difference is on the inside. We had a backup standard cartridge that we put in and now the printer is working just fine. So, what's happening?? Why isn't the XL cartridge working when we have used XL cartridges in the past just fine?

• No recent software/driver changes as far as I know
• Using a wireless connection
• Our printer is plugged into a power strip instead of in the wall. Does that really make a difference?
• I have been replacing cartridges one at a time rather than all 4 cartridges at once. Does that impact the situation? I thought that was just an urban myth because it sounds like a ginormous waste of ink (especially because we primarily print in black).
• Do I need to return/recycle our full XL cartridge, or should I be able to keep and use it?