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.
I'm having a problem with the ABS material not sticking to the build plate. I leveled the build plate and applied glue (glue stick) before printing. I even added a generous flange on the bottom of the part to give it extra area to stick to the build plate. Nothing works. After 3-4 layers it curls up so bad that the part comes completely unstuck from the build plate and starts to move around with the extrusion tip. I end up with a glob of ABS if I don't abort immediately. Should I be using blue tape instead of the glue stick? Any other suggestions?