Trying batch HDR PhotoMerge stacks in Lightroom error "Unable to Merge"

Hi all! I am trying to HDR photo merge multiple stacks of photos, but when I do so, lightroom tries to combine each of the stacks together, and results in a "unable to merge" error message.

I've watched multiple tutorials online where people are using Ctrl+H when they have multple stacks selected, and Lightroom automatically creates MULTIPLE photo merge tasks... Lightroom is not doing this for me, but rather combines all stacks into a single Photo merge task, which fails.

Can anybody straighten me out here? I can't continue spending loads of time selecting each 3-photo batch and excecuting the photo-merge task one by one... it takes ages.

Thanks so much all!!!

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