It probably would do no harm to ream out the glow plug holes prior to replacement. There is every possibility you may have gotten just some sub standard production run plugs as well. The newer plugs have increased in resistance not burnt out so it's hard to say what the cause was from my perspective. Unfortunatly you will have to try another set to find out if it was just poor quality plugs or not over time. The recommended brand has had consistency problems before I believe. Now as to the warranty thing. You keep you sales slip this time and use it six months down the road with the plugs you are removing now. Thats of course if they have a one year warranty to get a spare set. I would not want to really encourage unethical behaviour.