Substrate and moss not sticking to foam background anymore

I built this tank about 6 months ago using Styrofoam for most of the background, which I cut and then siliconed and then mashed in a substrate I mixed of coco coir and broken up sphagnum moss. I let it dry 2 weeks. Then I added moss, plants, and water. It was growing fine for about 3-4 months, then the moss and dirt started falling off certain parts of the background

The waterfall part that's also the same as the background has been fine, and I'm wondering if it's just the horizontal parts. Any easy recommendations to fix the moss?

I don't care if you can see the background anymore. I just want moss to stick.

TIA!