I had similar problems with using a gamepad and I finally got an answer from Epic. Wes Bunn provided a template project for me to download that I was able to go through and figure out how to use the gamepad with UMG. I am sure it will help you with your problem. As for the sliders, I think you may have to change that to something else like a button that will adjust the slider but that is up to you.
Check out the link that Wes Bunn provided in my post and download it so you can go through it and figure out how to set up your widgets. I hope this helps.