Acrolinx isn't always able to find the position of the flag in a document. If you click on a card in the sidebar and nothing happens, you've probably run into an example of this issue. The reason is usually due to something specific about the document, e.g. formatting, structure, embedded objects.
We can sometimes fix the problem if we have the document to reproduce the problem. If you see this problem occurring regularly, please send the document to Acrolinx support .