That is not only yourself, I have the same problem on and off when I plant something and after starting up nicely they sometimes die no matter I do.
But I am very impressed with the stubborn art of weed. it was about 3 weeks when we pulled out weeds in front yard but now there are again growing like nothing happened. That is so frustrating, I am not keen to use chemicals as it is not good for soil but mechanically to pulled the out also not the solution, it does not last long. Recently we read about ground cover thyme and decided to try in our back yard, that looks amazing, now they are blooming in purple and the ground fully covered, there is nice intense thyme scent and also a lot of bumble bees and bees around. That was a good decision :)
You have a garden, I am very enviable. I live in a condominium and there is only a small balcony where we can grow plants. But it is hard to pull out weeds once every 3 weeks... I have never heard of "the ground cover", is different from a lawn? I think it is very nice idea to suppress weeds without using chemical substances.
By the way, I saw the planter that planted Komatsuna in this morning. There were a lot of insects like small flies on the soil. Mary gold may have been killed by them :(