This symptom likely indicates that your vacuum pump is not pulling enough of a vacuum to quickly and efficiently get the air out of your silicone mix. After your mixing container is placed in the vacuum chamber and the vacuum pump is turned on, the rubber should rise, break and fall in less than 5 minutes.
Minimum Vacuum Pump Rating: Your vacuum pump should be rated to pull 29 inches of mercury (also measured as "-1 Bar") at 8 CFM. Any less than this is counterproductive, as you are enlarging the entrapped air bubbles rather than evacuating them.