xes, which version are you using?

I grepped through the sources and did not find occurences of 'impossible to retrieve'!
Do you get this in fMMS GUI when touching that MMS?
Since when do you have this behaviour?

Do you have a file named 'headers' in that directory? What is the status of 'Retrieve-Status'?

I had that once (long before) that MMS was downloaded correctly but somehow not marked as downloaded (in mms.db). I guess it had to do with a wrong header decoding at the end (some ending char missing) so decoding took place but it was not entered into db as downloaded/received.