Why dont i digest lettuce?

sometimes, usually when i eat at the dennys diner, i will order a burger and and the next day when i poop there will be bits of lettuce in my poop. ive tested this a number of times and it almost always happens. but when i buy lettuce from the grocery store and make a sandwhich or burger for myself, no lettuce in my poop later. but yesterday i was at a baby shower and ate some little home made sandwhiches there, and just about 10 minutes ago...lettuce in my poop! whats going on?!