I see only new builds appearing for Ubuntu 19.10 and 20.04 but not for 16.04 nor 18.04. Are those versions not supported anymore? Or is it simply a glitch in the build system?
18.04 (non-LTS) is end of life and all the repos were removed to the archive servers recently. If the build job uses a standard docker container, for example, it is likely to fail. The paths would need to be updated to point to the archive package repos now to build for the older OSes.