I've had some reports about modRana not retrying failed downloads. There are basically two download error types - fatal and temporary. The fatal types usually happen if the tile server can be reached bud outright refuses to cooperate (returns 404, 503 a something that is not a tile - usually an error page). The temporary errors usually happen if the server or network connectivity becomes temporarily unavailable. These errors should be recoverable and modRana should retry those tiles after a while. But I haven't looked at the code responsible in quite while, will have to check if it still works.

That's weird - once the tile is flushed from the cache its state is pretty much reset and the whole down-/load procedure should run with a "clean" slate. What if you quit modRana and then start it again ? Does it download those tiles or still has issues with them ?

You can also enable Print tile cache status to terminal and Tile loading status to terminal in Options->Debug and see if the output sheds some light on the issue. Also for some download errors it reports the tile URL it tried to reach so you can try if you can download those URLs by wget, browser, etc.