Since 20% glycerol is more than enough for storage of bacterial stocks, the problem is probably due to the extended time it takes the liquid to freeze at -20°C. The recommended storage temperature is -80°C. Some people even flash freeze their stocks in dry ice/ethanol or liquid nitrogen before putting in the freezer. Also, 100 % glycerol is very hard to pipet accurately. One method that works well is to dilute your glycerol to 80% (maintain sterility) and then add the appropriate amount to the culture to give 15% final glycerol concentration.