.... if the resin is still flexible after a proper curing time , you need to take a look at your mix ratio. Resin is very unforgiving...if your off even a few grams it can have undesired effects on the resin...especially the smaller the pour.
Increasing the harder (catalyst) will only make a bad situation even worse....the catalyst will be the clear bottle.
Be sure and shake BOTH the resin and catalyst before mixing...a couple minutes for both parts ("A" & "B") will work. Be sure and fallow the mixing instructions, especially the time duration.
To help insure a proper mix ratio, I pour the catalyst into the cup that I weighed the resin in and then mix since the catalyst is a thinner viscosity than the resin and is less apt to stick to the side of the cup like resin would.
Also Amazing Casting Resin is a by weight or by volume resin - if weight isn't working right for you, try backing it up with a volume measurement and see if the ratio is off. And like Brian said, making sure both sides are shaken up before mixing is a very good thing to do as parts of each can settle to the bottom and need remixing before measuring. My rule of thumb is if I haven't used it in 24 hours, then I make sure to mix each side up thoroughly before measuring. Another thing to look at is your working environment, was it a little cool before and after when you mixed the resin? A warmer environment will speed up a cure while a cooler one will slow it down. Also if you're using a mold, warm up the mold first will help the resin cure right.