On 2015-10-14 09:20:54 +0000, Hypatia Nachshon said:

I am a big fan of Hebe, esp. "Wiri Blush". Have them all over the garden.

Here's the weird thing, for example: Three in a bed; two will die and
the other flourish. Has happened several times. They all get the same
conditions. This happens sometimes a year or two after planting.

Can one plant be inherently "feeble" and another not?

Could be. Have you considered propagating the survivors via cuttings?
I hear it is easy with hebes.