Slugs and snails can climb fences and trees, nothing is safe from them. Barriers do help but as you noticed snails are persistent.
What works best is to let their natural enemies take care of them. They still won't disappear, but they do a better job.
I buy a ton of slug bait because, I don't have a lot of snail predators. I have a few cannibal snails, but they eat each other too, and I don't see a lot of them around. Toads (I don't have any), chickens (hens only, roosters cause too many problems).
Chickens actually do a good job of controlling snails, slugs, grasshoppers, geckos, and other bugs, and they will eat a few earthworms too. When we had feral chickens in the garden, we only found snails and slugs under pots the chickens could not get to. However, roosters roam with their harems and they crow anytime day or night and they poop everywhere. You have to cover seedlings, they are especially fond of corn seeds and lettuce seedlings. They hide in the bushes in the daytime and fly up to the low branches of the trees to roost at night. They will also compete with the feral cats for cat food left by feral feeders.
The problem with using sluggo:
It is less toxic than methaldehyde but it only lasts a couple of weeks, slugs lay eggs every three weeks.Sluggo needs to be stored in a metal container because it also attracts rats and birds. They will eat it before the snails do and come back for more.