Performing a Cup Test
There are many variables that can affect how a material performs and the outcome for your project. Proper mixing of Parts A and B, surface contaminants, using the right release agent in the right amount and humidity (in the case of urethane) are just a few.
Getting Down to Basics - eliminating all variables and performing a small scale test by mixing just Part A and Part B is a good way to start. We call this "the cup test". It simply involves mixing Part A and Part B as directed, pouring the material into a disposable polyethylene cup (do not use Styrofoam or PVC) and observing what happens. If the material cures as expected in the cup, you can conclude that there is something affecting the material when poured or brushed over a surface, into a mold, etc.
For example, if you find that your urethane casting resin is not curing properly when mixed and poured into a newly made silicone rubber mold, perform a cup test.
If the material cures properly in the cup, then you know there is something in the rubber mold that is interfering with the cure of the resin.
Another example, your rubber mold making material is not curing properly when poured or brushed over a surface. Perform a Cup Test...Mix and apply the rubber to the inside of a disposable polyethylene cup. If the rubber cures as you would expect it to, you can conclude that there is a surface contaminant interfering with the cure of the material.