We have had a beautiful section of yellow archangel in our yard for many years. The last two summers it is very sparse in some areas. Strange, since it is often invasive! Does anyone know why this might happen?

Has it grown much shadier there?
I had that and it looked very aggressive around my dogwood, But as the dogwood and shrubs have grown , the ground is very shady (instead of light shade or part sun) and my archangel is disappearing.

Thank you for your prompt answer. I don't think it's too much shade in this situation. It's a little bit sunnier there after neighbors' tree work.
Maybe the heat & dry spells we've had in NE Illinois are the culprit, but it's been happy for 20+ years. Go figure!

I am still surprised about mine too.
I had a patch of another Lamium, the usual blue flowered, that about totally disappeared over one winter. Maybe there's some thing about Lamium.
Being too dry sounds like a good theory.

I just looked again. The archangel is doing fine in deep shade. It is not spreading the way it used to with more light and less competition.