Several ggplotly blank in flexdashboard

I'm building a flexdashboard with interactive graphics using the plotly package. I'm using the ggplotly function. I didn' have any problem until recently when several graphs suddenly turned blank. These are always the same graphs. There is no problem with the code, i.e. it runs without errors and I generate the html document. You can see a screenshot of my dashboard below. As you can see only one graph is displayed. This is a large document and I'm only showing you a screenshot from one of seven tabs.

When I copy and paste part of the code into new .Rmd file and run it, everything is fine as the screenshot below shows.

What could be the reason for this behaviour?