I have had no tasks on my main computer (2019 iMac). My older computer (a 2015 iMac) had tasks until recently but now it too has none. I have unistalled ,reinstalled, reset, etc. the whole 9 yards, but I'm getting nothing and no error messages. What can I do?

You should check the status on the project sites. For example Rosetta: there is little or no work lately, check https://boinc.bakerlab.org/rosetta/forum_thread.php?id=8439&postid=113180

On Boincstats there is also an status page for all active projects, check https://www.boincstats.com/page/projectStatusInfo

Thank you. Most of my recent jobs were with World Community Grid, and I've managed to get work from there again by connecting directly to it rather than using BAM.

WCG has been very intermittent since they moved hosts. I'd recommend at least one or two backup projects to ensure you never run out of work; I have no fewer than ten projects attached to all of my hosts and they never run dry!