Okay, this is not the most attractive solution but duct tape works. . .honest!
I gave up over a year ago trying to glue my step and decided to tape over the seams on top. It works like a charm. After 6-8 months of usage the tape started to wear down so I tore it off and put on new tape. Just make sure when you tape it that you make it long enough to cover the entire top seam and wrap around the bottom. I taped the right and left side of the step and then noticed the front and back sides were also starting to peel so I taped those as well.
I admit this solution is not for everyone. Some may not like how it looks, but it's functional and cheap.:7
Take care,
Sue