While I am not sure why your martagon is declining, I find the martagons that I have on my property tend to do better in a more shaded site. I have two different varieties planted in a north facing bed that are thriving, while those planted on the east and west facing location seem to get too much sun and heat. So I would be inclined to try a different site for your plant; I know I will be trying a more shady situation for those plants in my yard that are not performing so well. Good luck!

I could try that then. I had no idea that 6.5 h would be too much.

Well, I have read that martagons will do well in full sun but I have found they didn't for me. That may or not be true for your conditions. My other thought was your raised bed; not sure if that would contribute to any problems. Because a raised bed warms up earlier in the spring causing earlier growth of plants, a cold snap could damage that tender growth which may affect the long term size. This is speculation on my part. I think either way, if your plant is declining it can't hurt to try it in another situation. It may "sulk" after being moved; I think that is typical for Martagons as well. So I guess you need to choose what's best for you. Hopefully some others wifi chime in with some advice for you.