It might be intentional but two of the objects are yellow and can be confused. Make one orange?
I'm guessing that you will introduce more objects in later levels so that it is more difficult?
You could gain a box to put objects in after each 10 levels completed (to cope with more objects being matched)?
Or you need to start matching 4 objects in later levels?
The scenes could change ever so often? A space one, lava field etc.