Using MAS epoxy

From time to time I have used some fast hardener for small jobs, cheek plates ect. The problem I'm having is the stuff is not getting hard enough to sand. I have had no problems with the slow hardener, in fact it is sandable before the fast. Also I am using a container graduated in CC's for mixing and am mixing for 2-3 min.. so that is probably not the issue.

Has anyone experienced this problem. Do you think the hardener is bad ?