Question: Have you ever experienced problems with delay in setting in the presence of ground granulated blast furnace slag or fly ash and water-reducing admixtures, especially using imported cement?
Answer: I have experienced a complete failure to set with a particular combination of class C fly ash, water-reducing admixtures, and a particular cement. I suggest you contact the admixture supplier and don't take no for an answer. In my case, changing either the fly ash or the cement would have fixed the problem, as would a different type of admixture.
I believe lots of people have experienced similar problems. GGBFS or fly ash (depending on the class), is likely to contribute to the delayed set time, depending on the percentage of cement replacement. A particular cement may or may not contribute. Water-reducing admixture may or may not contain ingredients that cause delayed set time. It is also possible that there is interaction among the three ingredients. The best solution is to test them all in advance either individually or together to determine what's causing the problem.