This issue was found for the first time when I converted flac files to mp3s. I waited for several minutes but 'conversion complete' or close butten didn't come out.
When I converted, I closed all other processes which can give influence on conversion.
I just used the built-in encoder with CBR/ABR encoding, 320k option. Fortunately converted files are all played well.
And, recently I found how to make sure of conversion completion. That is to click 'details' on action window of conversion progress and check the status field from the right side pane(see screenshot).
All of my flac files have the tags created by MC Analyzing and those of title/artist/album/track/genre/comment/year/lyrics made with MP3TAG.
I tested more with other encoders and found conversions are all successful, but sometimes 'conversion complete' and 'close' butten normally comes, sometimes not come out regardless of files and encoders.
quite random