Thyme Does Not Creep

The creeping thyme under the white bowl has not spread one bit this year. It's green and healthy, but not filling in any bare spots where I had removed perennials last fall. 

Perennials got overrun by the creeping thyme

When I had tried to grow perennials -- blanketflowers, sages and blackeyed susans, obedient plants, dwarf agastaches -- in the carpet of thyme surrounding the birdbath, they failed. 

The thyme overwhelmed each, growing right over each little emerging perennial every spring. The creeping thyme completely choked everything out.

So I removed the plants, letting the thyme spread where it will.

But.

No action at all this year. I replaced the birdbath with the white bowl. I watered in the bare spots all summer, added compost, encouraged it. It hasn't spread anywhere all summer, not even an inch.

All summer it has looked healthy but has not spread

Why did it spread so quickly and aggressively before but has stopped at its existing boundaries this year? I'm puzzled.