Which version of Lightroom are you using?
Are you merging raw images? If so try using other file formats?
Try creating a new catalog and test them same.
Thanks Akash. Turns out that my computer's 6G of RAM is insufficient to process a 5-frame HDR bracket in Lightroom 6 at 35 megapixels per frame. If you open your task manager and go to the performance tab, you can actually see the RAM usage in real time. Looks like I'm looking at a new laptop With 16Gb of RAM and faster processor. Mine is 4 years old anyway. Thanks again!
I have just 8 GB RAM, and have been able to assemble HDR images from five separate images. I have compared those 5-image HDR files with 2-image and 3-image files, and see very little (if any) improvement by using all five. Your results may vary, but you might try reducing the number of images until you get your new laptop.
Thanks Jim. I've figured out the problem.