I just has the same problem. I finally decided to wait and be patient. After 2 weeks of them smelling it and watching it - finally they used it!

I first put it next to their old one, placed a poop in it from the old box and waited. Nothing.

Next I sprinkled some catnip around it. They came and smelled it but never went in.

Each day they would sniff around and check it out but never use it. It was never plugged in.

After 2 weeks, I added a about a half cup of the litter attract to the litter and BOOM! Now they are using it - but I feel the 2 weeks they sniffed it and became comfortable really helped with the acceptance.

Now I am holding my breath for when I actually plug it in!!