We use the Tommee Tippee sticky mat and bowl, but not sure if that would be too small of a bowl for your LO?
Have you tried wetting the bottom of the mat you have before you put it on the surface? The first time I used the Tommee Tippee one, it didn't stick, but I either ran the mat under water or gave it a wipe with a wet cloth and tried again - stuck like glue!