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 runnersI 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 When the runners on the doors broke recently, I called Kohler about replacements, only to be told my guarantee is no longer valid as Kohler has taken over Daryl.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, CarshaltonYour tale is worryingly f