No hot or cold water from bathroom faucet

My neighbour is an older guy who isn't very handy and he asked if I can do him a favour. His bathroom faucet was dripping slowly but constantly and he asked if I could have a look at it as I'm an apprentice plumber (no service experience thought, just over a year building highrises) I figured that the rubber washer on the end of the cartridge was likely worn so I said no problem. I took both cartridges out and replaced the rubber washers (they did look ok but had a circular indent on them, I assume where they seat)

After reinstalling the cartridges, only the tiniest dribble of water comes out of the tap for either hot or cold. If I pull either cartridge and turn on the stop in the cabinet, loads of water comes out the hole the cartridge was in.

Ideas???

You do not repair a cartridge !
You replace it !
The worlds first plug and play unit that came along in the 70's to be easy to fix just by
replacing the cartridge

How about removing the aerator from the end of the faucet and cleaning it or replacing it? Bad
"cartridges", and I assume these are American Standard AquaSeal stems, ONLY prevent water from one side or the other, and it would be unusual for BOTH to fail at the same time. IF you turn on the cold and only get a little bit of water, and then turn on the hot and the flow does NOT increase, the blockage is PAST the point where the two come together, and the aerator is usually the only spot that happens.