I have two Android phones, one is Android 7, one is Android 6. The Android 7 phone joined 10 projects, and runs them all fine. The Android 6 phone had trouble finding any projects at all, and eventually I got 6 attached from the master list within the standard Boinc client (same version on both phones - 7.4.53). The other 4 (Universe, World Community Grid, Enigma, and Einstein) do not appear in the list to join, and if I try to join by URL, nothing happens. No failure notice, nothing, it just doesn't add the project. Any ideas?

I have currently 3 cellphones crunching. 2 of them get tasks non-stop. The last one, running Android 4.4.4, didn't.
It appeared that all I needed to do, was log out of my network, and log in again.
It then loaded projects from Einstein.

I did get a notice from World Community Grid, having their servers offline, however, I never get any projects from them on my cell anyway.

Enigma has not had ANY tasks for any platform for months now.
World Community Grid (WCG) has not had any Android tasks for about 1 month now and in past week sent Android users an email indicating that it will be an undetermined amount of time before any work for the platform will be available.

Enigma also doesn't allow new users to register.
They should update their servers to a 'down' status.

I have no issues running Boinc on Android 4.4.4, 4.4.1, 6.0.1, 7.1, 9.0 and 9.1.

I have no issues running Boinc on Android 4.4.4, 4.4.1, 6.0.1, 7.1, 9.0 and 9.1.

It worked eventually. But initially the project list (on the 4.x phone) was empty. I was asked to select a project and given a blank screen. I guess it takes 20 minutes to find the list.

And there really is something dodgy about this phone. Do you find you get full or half page ads in front of any app you're running on Android 4.x? The Android 6.0 phone doesn't do this, I see zero ads. I even reformatted the older phone and the ads went away, but install a few little harmless apps (which google play claims they check!) and again, ads every 2 minutes. And more irritatingly, 25% of the CPU taken up by who knows what, leaving less for Boinc to use. No virus or malware scanners found anything. And I can't find the simplest of things for it like I have on a desktop - a task manager that shows CPU usage per app, so I can tell what's eating the power.

In my case, I uninstall all software, because most Android 4 phones have only 500MB of RAM, thus are very spare on resources.
Especially browsers.
I take the background picture away, and replace it with plain black (or a 1pix or 4x4 pix image of any color I prefer, and stretch the image over the screen, just to save a few bytes of memory).
CPUZ does show ads on Android 4, but Android 9 I don't see as many.

I also disable notifications for all programs (a tedious process).

Under Android 9 you can set which apps are allowed to use Wifi or Data. Of course default everything is on. A lot of apps on my phone I disallowed having WiFi or Data access for, which killed their ads. Doesn't work on all apps, some use a different technique. For older Android versions you can use firewall apps, such as Netguard - which has the added upside that it doesn't need root either.

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That's why I said, use a firewall app. For instance, Netguard is ad-free and doesn't require you to root your phone: https://play.google.com/store/apps/details?id=eu.faircode.netguard&hl=en, although it may require Android 5.1 and later. Ad-blockers usually only work on browsers, not as stand-alone apps.

https://play.google.com/store/apps/details?id=app.greyshirts.firewall&hl=en

Requires Android 4.0 and up

Else check through https://techviral.net/firewall-apps-for-android/

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I wonder if your router can block ad related websites?

Sidenote: Android 10 is now supported by Universe@home.
Hope that more projects will follow.

Sidenote: Android 10 is now supported by Universe@home.
Hope that more projects will follow.

I've never seen a 10, they actually exist? Unlike Windows 10, where almost every computer has it, I find brand new phones running 6 or 7, not sure why Android OS is like this - does new Android not like phones that aren't super powerfull or something? I have a 4 and a 7, both purchased in the last 4 months, both run Universe, Asteroids, LHC (only sixtrack which is very rare to get hold of WUs for), and Yoyo. I can't seem to get any other project to give them tasks.

Android 10 came out around November to January, for Google Pixel phones only. In February, my Essential phone had received the update, and newer Samsung phones had the update in March/April.

You're not missing much.
Android 8 and 9 never really was a thing. Most phones with these operating systems, got the free Android 10 upgrade, which I'm hoping is an LTS release.