Why won't a shower firm honour its lifetime guarantee?

I bought a Daryl shower but Kohler, the owner, says it won’t
replace its broken door runners

I brought a Daryl shower manufactured by Kohler
with a lifetime guarantee in 2007. When
the runners on the doors broke recently, I called Kohler about
replacements, only to be told my guarantee is no longer valid as
Kohler has taken over Daryl. I don’t understand as the lifetime
guarantee should be honoured whether or not the company has been
taken over.
GS, Carshalton

Your tale is worryingly familiar. Twelve months ago CF of
Ipswich, who had also
bought a Daryl shower
in 2007, was told that the guarantee was
invalid for the same reason.

Source: FS – All-News2-Economy
Why won't a shower firm honour its lifetime guarantee?