Vegetation is not generally a major part of a young yemems diet, some do, some dont.
Wild ones resort to vegetation when live prey is scarce, or/and incidently when drinking from leaves etc.
Like many reptiles, they are generally insectivorous, becoming omnivorous as they get older/larger, as the need arises.
Dont stress if it dosent bother with it. A captive well fed cham dosent need to eat vegetation.
Incidently, most lettuce is of exceptionally low nutritional content being mostly water.
Its not even good gutload. Try asian Greens, kale etc. Buk choy is a fav.
Baring that get some live pothos Epipremnum Aureum or/and umbrella plant shefflera arboricola. They seem to like it from time to time.
The best way to get vegetation into your cham is via live food (gutload).
Does anyone gutload with the above plants (from trimming?)