Don't give up, change something like less lettuce or milk and get some more bugs.
One of my batches died after a really hot day, after which I moved them to part shade. They then ate lots of the really green algae in the bottom and it didn't regrow but now there is a population of brown algae stuff that they eat anyway and that doesn't seem to go away. I think it takes a little while to get the bugs in the right spot with the right conditions.