British Columbia: Trifolium Repens Atropurpureum; Dark Dancer

Although Dark Dancer can be quite invasive, I have the ideal spot for it, an area of 65 sf on
the west side of the house, completely surrounded by house, concrete walkways and such,
where it served as e beautiful background for pale green hostas.

Unfortunately after thriving for 5 or 6 years it suddenly started to become patchy last year
and this year the Dark Dancer has all but disappeared. I am at a loss as to what could have
caused its demise. As there is no obvious indication of any disease, I must assume, that it
is now lacking some nutrient. What does clover need other than regular fertilizer?

I would welcome any advice.

Thanks in advance
Olaf