He probably hasn't forgotten. Whether he wants to cooperate or not is a different matter, so use some treats to sweeten the deal while you re-establish the behavior.

It's probably just like your art contest picture shows so well, he doesn't have great capacity to link things together yet.
Do budgies love millet spray just like tiels do? If they do, you can't fail with that. Unless you are one of those 1% unfortunate bird keepers whose bird doesn't like to gorge on it!
When I watched the treat-training videos for the first time I thought "Yeah right... It can't be so straightforward". Then I found out that it is. They NEVER forget that
They remember the oddest things, IE. colour-coded food bowls even if you don't use them for months, so I can't believe that Poppy can forget the training just like that.

He doesn't really like millet much. I don't know if he know what it is yet though. I have it sitting in his cage and he just ignores it. I taught him to step up without millet and only eats it if he sees Beck do it. And when he does it eat when he sees he takes very tiny bites like he's doing it to be polite. He doesn't really care for it otherwise.