Kinda late to the party... the chip gets stuck, and doesn't let you swing open the shelf holding the toner.
Use a screwdriver... carefully slide the chip towards the toner cartridge, then swing the shelf open.
I've tried it turned off, plugged in, unplugged...those two cartridges won't come out. The machine is basically brand-new, that is we've had it for over a year, but we don't use it much and this is the first time we've tried to change a cartridge, so the cartridges that are in there are the ones it came loaded with.
Is it unplugged? It sounds like they are either still engaged or the release switch is broken, which isn't likely with multiple toners.
I have a Workcentre 6015. I'm trying to replace a toner cartridge but the old empty cartridge won't come out. The yellow comes out and the black comes out, but the cyan and magenta won't. They come loose, but then they're stuck half-way out. What's the trick?