Super frustrating!
Hard to say what the exact issue is without a link to the page you are trying to optimize. Could be a cacheing issue if you are testing before images have propagated. More likely, there may be other background images on the page that are still getting flagged. Also, don't let this automated checker replace common sense, they are combing and automatically throwing out suggestions - if you have compressed an image per their guidelines: https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/image-optimization
then start to focus on the other low hanging fruit if you know that image is already optimized (usually they will call out your JS loading - Google Tag Manager GTM can help with some of this).
You can also use this tool from Pingdom for page speed suggestions: https://tools.pingdom.com/
And if you are serious about performance, look at New Relic for the actual performance stats on the server side for your web app.
GL!
