A few things with that and a few other casting products, first, it never seems to harden like other cast materials, also if you are adding any pigments, they can wreak havoc on the set up...
also it can take several days to harden and depending on the atmospheric heat and humidity it will also affect the set up time.
another thing is age, if the product is older than a few months then its past its life or if it has been opened for some time it tends to be hygroscopic and will absorb ambient moisture.
these are things i have had to overcome with my experience in casting and mould making.
i have the same key set that you do and if i find time i will make a silicone mould of it and see what can be used to make the key assembly, i have a product in mind that will be very similar to the oem material.