Most dogs DO show discomfort when having their anal glands expressed, even if they are not impacted. As a groomer, more often than not I am holding on to the tail of a 'bucking bronco' when I express glands. It's by far what dogs complain the most about, and if a dog is going to scream or bite for anything, it'll usually be the glands.
The only time that I find my dogs actually have anything in their glands at all is if their food is not quite right for them. That's one way I know a food is not quite agreeing with them. I've finally got everyone on a food that works for everyone. Nothing comes out when I express their glands -- Visa's actually makes a little "pffft" noise because only air comes out LOL. It's been two years since I've been using this food and still the glands are empty. I told this to a friend of mine who was having real issues with her Belgian, to the point that one glad ruptured. She switched the food and the dog no longer needs glands done regularily.
Try switching her food and seeing if that helps.