I would always guess one of the following scenarios has occurred:
- Was not left to cure long enough - the rule is the smaller/thinner the thing the longer the cure for most resins
- Humidity or temperature are not right - too humid or too cold/hot - or temperatures in environment are changing too much - most things want low humidity and 70 degrees
- Inaccurate mix due to ratios or mixins - mixins being pigment / dye / ink
- You are using old materials - they have a limited shelf life, if they change color - they are going off or have gone off
I also recommend you do not come into contact with materials that are curing. I have seen something that is halfway curing just go off the rails because someone touched it at some point in the process with their actual fingers. The environment in which it cures is crucial. To give you an idea I always cure in my garage - I live in Socal so I have lower humidity and a more consistent temperature in my garage than in my house. My house is likely to have much more humidity due to all the water usage inside.
I have had things take longer to cure or never fully cure when the above scenarios have happened. A great way to test these out is to purchase a silicone mold and test with it. I have one that is an object I love using for pour off, meaning extra materials are used in this mold so the plastic does not go to waste. You can use this to validate your process and materials.
I do not use a scale but I eyeball using measuring cups, rice, and a permanent marker. I use easy cast clear resin generally for my projects and have had great results. Most of them are not key cap based and are more sculptures I have made and then cast. The keys I have done have been the WORST to cure, they are thin and the materials have to have a chemical reaction to harden, the less material the less reaction.
Also make sure you use two cups for part A and part B - do not mix Part B into Part A but rather poor A into B. Don't try and be cheap about it, scrape the sides.
Also ensure the gloves you are using are made of nitrile. If you are using anything but there is a high chance that you are causing the chemical reaction to fail. They are sensitive and you need to make sure you take absolute care when dealing with the reaction you are creating. Although they do not discuss this often in the terms of chemistry, this is actually what you are doing - a reaction between two chemicals.