x-no-archive: yes

I have a Gerber single handle shower faucet, model 49-700, it leaked. It has
two washers, one for the hot and one for the cold. I wanted to buy new
washers, but I was told they weren't available and I had to buy the
cartridge. So I bought the cartridge and used the old spindle. When I got
it all back together, I could change the water from hot to cold, but I
couldn't turn it off. The spindle goes through a big plastic piece, and
through a spring with plastic washers on either end. I thought the spring
wasn't compressing enough, so I tried stretching it, but that didn't help.
Luckily there's two other spindles for a normal faucet, I turned those off.
I don't want to have wasted money on the cartridge, why won't it turn off ?