Whenever I have a number of items in the queue to be converted, sooner or later Calibre finishes converting one book and fails to start another. All books still pending on the queue are listed as "waiting" and no error message appears. This happens regardless of whether I am actively using Calibre while it is converting, or have left Calibre in the background while doing something else, or have left Calibre in the foreground and gone away for hours.
The only remedy seems to be to quit Calibre and restart it, then find all the books I want converted and put them back in the queue.
This problem is extremely consistent, and will happen any time I place more than one or two books in the queue.
I'm presently converting from various formats to mobi, but I previously had the same problem (perhaps less consistently) when converting various formats to epub. The input format doesn't seem to determine whether the problem occurs either: I could convert five books from lit to mobi with no problem, and then a sixth lit book from the same source won't start.
Thanks for any help you can provide; I really do love this app despite that I have a number of small difficulties with it.