Well, that’s a big part of the problem there. Plastic is characterized by having a soft non-colored highlight with a lot of diffuse showing through. Hence, if you don’t have anything in your environment to reflect, you’re not going to get interesting reflections, and it won’t look like plastic.
First off, I suggest making a single lego in the middle of your scene with nothing else, get that looking the way you want, then worry about adding other blocks. Next, start adding planes with a self illuminated material on them to the scene, these will be the reflection cards. If they don’t show up strong enough in the reflection, add an output to their diffuse slot and change the rgb level to a value greater than 1, see
the pottery example is a good starting spot.
As for where to put the cards to get the best effect, that’s a matter of trial and error. No perfect formula, however, one piece of advice is to place them such that they define the shape of the object, as in, you want your object to seem dimensional and 3D, so place the cards such that their reflection helps give the object depth, like for example having one side of the lego have a reflection, and one without a reflection, that will help the lego look like it’s not flat. Also, you may wish the chamfer the edges of your lego, since no real world object is ever perfectly sharp at the edge, this will also give a nice area for reflections to appear, so you can get nice white lines at the chamfered edge.
Lastly, try and find a photo of what you want to do, or even make one with a digital camera and some lego, then analyze that photo, what makes up the scene, then replicate that in 3d. It’s always easier to make something in 3d if you have real world reference.