This also happens in high temperatures. They tend to go dormant when it's too cool or too warm.

Clivias typically put out leaves in spring and/or fall. On a mature plant you can expect, on average, 4 new leaves each year. On seedlings, less than that in the first year or two. I think the young plant is concentrating on roots, not leaves. After that, they start picking up speed. At least that's been my experience.

I agree with Gail. Clivias take a while to grow past that 1-2 leaf stage. Just be patient. Also pull it out of the ground and make sure its roots are not rotting or bursting. Sometimes slow growth can be atributed to root loss. The plant will look ok but be battling for its life under the ground. If that is the case lighten up your potting media with something coarse like a heavy pearlite or coconut husk.

-Jon

Mine grow best in the winter on a northern windowsill (double paned in my case). They stop in the summer.

I meant, when they are young; when they are old enough to bloom, they don't grow in the winter, of course.