Why are my boosters locked?

So I know this is was answered already, but to add to this for others who also look up the same issue. I hit the same level and had them padlocked. Found your question about it and then kept playing. I found that this and the next level had locked boosters. They were both simple levels and then I realized the first one introduced a new game mechanism, and the next level with no boosters used the same new mechanism. The 3rd one my boosters were unlocked again and it was a typical difficulty with the new mechanism. I think the purpose is to introduce you to a new mechanism with very simple levels, and once you have exposure to it, they give the boosters back and revert to more difficult levels.