They grow perfectly well here (and in Portland) if given the right conditions. Corsican mint is a bit finicky in its establishment, sometimes dying off partially where originally planted but often popping up elsewhere.

How much sun does your flagstoned area receive? And what's currently under and between the stones - sand? Gravel?

gravel under. back filled with native dirt in prep for stepables

south side of house. not much sun this time of the year but gets hot there during the height of summer

Of the three choices you tried, the Leptinella would work best. Corsican mint does best in shade and with consistent moisture and the elfin thyme appreciates a full sun, dry environment. Leptinella (NZ brass buttons) will take both sun and shade and is not too fussy about moisture once established. But there are other low growing, mat-like GC's you could try - rupturewort (Herniaria glabra) or Corsican sandwort (Arenaria balerica). Blue star creeper could work as well if kept watered.

The cracks between the stepping stones get way more water than if the
stones weren't there, plus every time the walkway is washed off. That's
a perfect environment for all sorts of weeds.

The pictures in the garden magazines don't mention all the work it takes to maintain 'stepables' between stepping stones.

I use glysophate as needed so nothing grows between them. I used broken concrete. The price was right.

http://i75.photobucket.com/albums/i283/Botann/My%20favorite%20pics/2570384470036511179.jpg

Mike

Great feedback. Thanks, all - especially on the weeds. I'll look to other options.

small sedums are working for me in Washington state.

Lime thyme does well for me in part and full sun and isn't fussy about water. It forms a thick, low mat. I have a patch of it that I've taken chunks off to edge other areas. I do this in fall or early spring and they survive fine.