They are very common where I am (Norfolk, UK). Fields get ploughed up, they spray the fields with stuff but the poppies can still come back. There quite a few growing on the edge of the field by our veg plot.Lovely, I had such a poppy in my garden this spring!
The Flanders Poppy should be the same one as I'm calling the Corn Poppy. Common names do vary and the same names can refer to different plants... but that (Papaver rhoeas) is the one that grew in Flanders.I have sown some poppies as well, they meant to be flanders ones on anzac day.
Perhaps someone in your area might be able to advise why yours might be struggling. My own UK experience at is that they do reseed themselves very well - almost too well - your problem more likely would be they are popping up in places you don't want them. Another thing with these is the seeds can lie dormant for years.I planted two red poppies in my front garden about 3-4 years ago, but only one ever blooms. I was told they do reseed themselves very well