Layers do not appear intermittently

I've got an app in which the layers do not always appear. I can use the identify tool to select them when they are invisible, so I know they are there, but they fail to draw about 50% of the time.

Developer console shows

"Uncaught (in promise) Error: A listener indicated an asynchronous response by returning true, but he message channel closed before a response was received"

"[warning]: Error fetching legend information: RequestTimeoutError: Timeout exceeded" in loader.js:470

How can solve this?