Thank you all for your comments, they were all very helpful. I resolved the problem by spreading the glue stick very thin with a wet paper towel, then turning up the build plate temp to 110 C (it was at the default ABS setting of 90C). With all the suggestions to run the build plate at 110 C, I think Ultimaker should change their firmware to make this the default ABS setting rather than 90 C. The difference is amazing. ABS is working beautifully now. If I wait for the build plate to cool down after printing, the part comes off easily.