I don't believe that Merlin's is the problem. If the original pieces inhibited the curing, the rest of the mold would cure but where the rubber touches the plaster would be goo. Inhibition does not effect the whole mold.

Sometimes you need to get a paintbrush and actually stir the catalyst in the bottle because shaking just won't do it. Stir the catalyst a few times then pull the brush handle out and look at it. Is there a thick bob of catalyst on the end of the brush handle? If so, then your catalyst is not stirred up. Stir it like mad, then shake the bottle afterwards to help mix the thick parts together.

Thanks,
Bruce Hirst