Failed recordings that play but don't decrypt


I managed to record most of the episodes of 'Due South' shown recently, and I later set to decrpyt and transfer them to my PC for storage. But the last two episodes wouldn't decrypt, but they play perfectly well on the Humax. The first time I played each one, I got one of those 'Failed to record' messages, but not since.
Any ideas what the problem is?
If there is a thread that covers this, please direct me to it, I have looked but can't find one.

They won't decrypt if the protection flag has not been cleared. This is the norm for HiDef recordings, and the autounprotect package is there to automatically disable the protection flag. However, it is not unknown for StDef programmes to get broadcast with the protection flag, and I can't remember whether autounprotect was updated to account for this possibility.

How to tell: look at the recording in question using the WebIF media browser or the SUI Media button. Do they show the "Enc" icon?
I don't see any icon to indicate 'Enc', and I don't know what it would be, but the options offered include 'Decrypt'. This time, it worked on both episodes, so either I goofed yesterday, twice, or something has fixed itself. There is no doubt that they both flashed up the 'Failure..' sign when I first tried to play them on the PVR, so there was something not right. I notice that one of the episodes has no .htm sidecar file, and when I play it with vlc, there is no time scale, it shows zero at both ends. I saw this on at least one other episode, too. What is that file?
Now to work out how to get the two episodes that I missed recording!
Thanks for the reply, Black Hole, it's good to know that there is help out there. I recently fixed a failing HDD with the debug and troubleshooting info. on here, so I am much obliged.

.hmt contains all the tagging info about the .ts - media title, properties (HiDef/StDef, whether protected, whether decrypted, bookmarks...)
.nts contains indexing shortcuts to make skip and trick play work
.thm contains the thumbnail that is displayed in the media list

Without a .hmt (not htm - that's a web page), a .ts will play "naked", just as if it were any other media file imported rather than recorded. An encrypted .ts file cannot play without the .hmt to indicate that the unit needs to send the file through the decryption hardware before it goes to the renderer. If the sidecar files are missing, we have a process to regenerate them.
Ah, ok, thanks. I have seen comments about regenerating sidecar files, but a pointer would be appreciated.
But what I have seems to work, at lest well enough for archiving purposes. More information can only be better, though.
Why would a recording be made without the .hmt file?
Many thanks.