Euromillions and reliability of results

Here is a strange one. I play lotto and euromillions (EM) with some regularity, though not religiously. I was doing my early spring cleaning and I found three old tickets, two lotto ones and the third EM, all of them from the third quarter last year. I went online and to my surprise one had a €12 prize and the other (EM) for €9. So I took these tickets to the local shop and before handing them in I scanned them in the lotto machine, but only the result of one was confirmed. I handed both tickets to the guy at the counter and the same was confirmed. I asked him to print the results for the EM one back in November and we checked it together and indeed it was due a €9 prize, but he couldn't pay it since the machine didn't recognise it was due a prize. I went to a few different shops and the same, over and over.
I was told I might be able to get the money at the post office, but the problem I have is that in many occasions I used those machines to check and always trusted them - ie, I dumped tickets when I got a "not a winner" result. Now I am wondering if I may have lost prizes due to other "false negatives".
Anyone out there ever experienced this? What would you do if this happened to you?