Getting above level 6-10

I have made it to level 6-10, but I can go no higher, What is the key to getting
to level 7-1 & beyond. Thanks Snowman2677

Catching all the creatures. If you move your mouse over each world it will tell you how many are left to capture before moving on to the next world

What am I doing wrong? I go back and get to get more monsters and they are not adding up