How to use callJsFunction to call a method in a custom PolymerElement

I have a custom component (extends com.vaadin.flow.component.Component)
backed by a JavaScript PolymerElement.

I’m trying to use the callJsFunction() to call a JS method but
I can get no results. No exceptions, nothing in the logs in
either server or client.

Any exeamples or ideas how to debug would be appreciated.

wbr Kusti