Slugs don't always leave a trail, they are sneaky and come out at night and chomp your plants.

Could be snails either. You don't say whether the leaves have holes or are just looking miserable - lobelia do like to be kept on the moist side so they could just be too dry but unlikely this summer! Sadly snails and slugs like those moist conditions too. Try some slug pellets hidden underneath a stone or piece of wood in case it is them.