This is a common problem when the benders are worn or dirty.
Check that the formers and guides are clean and in good condition. If they are mucky give them a clean with wire wool. Also check that the rollers are free to spin. Also check the guides for damage. I was taught to keep my guides separate from the rest of my tools as they are easily damaged.
When I was at college basic maintenance of tools was taught as part of the course but I assume this is not the case now as one of my work colleagues had a similar problem with his bender and I had to show him how to clean it.
Mike