Instances "did not pass health check after command execution", but no errors in logs. How to diagnose?

I'm trying to release some code using "eb deploy". Normally this works fine, but when deploying anything that includes the code from a particular pull request, the deploy fails, seemingly without logging any errors. I see:

Instance id(s) '<a,b,c,d>' did not pass health check after command execution.

When looking at the logs, I don't see any errors from the time that the deploy happened, and in fact it says "Application update - Command CMD-AppDeploy succeeded".

The logs I'm looking at are the ones in the zip you can download from https://us-west-1.console.aws.amazon.com/elasticbeanstalk/home?region=us-west-1#/environment/logs?applicationName=MY_APPLIICATION_NAME&environmentId=SOME_ENVIRONMENT

(And I did specifically look at logs for one of the instances that it said didn't pass health checks)

Other than checking these logs, what else can I do to help diagnose this problem?