Where’s that greenhouse effect?

I know that conditions in a greenhouse in South West London in early May aren’t exactly like the tropical rainforest. But I thought my new greenhouse might at least grow things as well as they grow on the window-sill in the kitchen. Not so far.
 
Here is my first crop, started on the window-sill and about to graduate to the greenhouse.

They’re a collection of seeds that came in a gift pack, called — a little too cutely — “Psychedelic Salad”: unusual coloured beetroot, cucumbers etc.
 
And here they are in the greenhouse, ten days later:

The lettuces have all been eaten by something, the spring onions look anorexic, and the beetroot and radishes are all over the place.
 
 I have high hopes for the cucumber, which actually look quite healthy:

But going back to the window-sill question, more recently I planted some tomato and sweetcorn seeds on both the window-sill and in the greenhouse. So far, only the window-sill ones are doing well:

There’s no sign of the rest of these packets, which I planted in the greenhouse. I’ve always believed in the greenhouse effect, but in my garden, it seems a bit of a myth.