I've signed up for CPN through Boinc, but not getting any work. Is there something else I need to do? Is this project active?

There aren’t any Windows models available at the moment — apart from resends — but the last batch was only four days ago so you shouldn’t have to wait long.

I'm only running a couple Windows machines; most are Linux. I don't get anything on any of them.

The Linux machines don’t seem to have made contact under that account. It might be worth detaching and reattaching those machines to the project as some of the URLs changed a while ago. Other than that, it would be a good idea to check out the Linux threads in the Number Crunching section of the message board if you haven’t already done that: I’m afraid my Linux machine failed a while ago and I haven’t got around to setting up a new one — the new generation of Linux models seems to have provoked a lot of discussion in addition to the usual 32/64-bit library issues — they’re big models.

Thanks for the help! I'll check this out later. I've wanted to contribute to this project for a long time, but somehow forgot about it! Now I have a stack of beat-up cast-off computers to devote to it.

I'm only running a couple Windows machines; most are Linux. I don't get anything on any of them.

I'm not sure what I did to fix this, but I was having the exact same issue a while ago. I'm now running Boinc 7.9.3 instead of the latest beta and Ubuntu 18.04.3. Also in the Linux forum you'll find what 32 bit libraries are needed. I'd install them, detach and reattach to the project, and see if that fixes things. My recommendation is to wait a while between detaching and reattaching.
I remember on one machine I had to reboot and manually check for updates from the project, too.
Hope that helps a little.

In addition to that good advice, note that there is a one-hour back-off between attempts.
So if it doesn't get anything the first time, you have to wait another hour. Let it sit overnight, and it might work.