Little Windows Brilliant Resin is a proprietary, pure formula, which starts life as a solid. If it gets colder than it likes, it can begin to crystallize and thicken, and may even solidify. You can remedy this by heating your Part A, and the easiest way to do that is in the microwave. Remove your cap and microwave for 10 seconds. If not clear and thin, replace the lid and shake a bit to blend in any crystallized resin around the inner neck of your bottle, then remove lid and heat for an additional 10 seconds.
For safety, do not heat more than 30 seconds total as it may be too hot to handle. Allow it to come back to room temperature before mixing in with your Part B. This can be repeated as necessary.
WARNING: DO NOT MICROWAVE ANY OTHER BRAND OF RESIN OR TYPE OF CHEMICAL, AS THEY CAN RELEASE HARMFUL GASSES. Little Windows Brilliant Resin formula complies with FDA standards for food safety, and does not contain any solvents.