Some players have found that, after successfully completing the first 5 levels of "Bulking Up" in Cranky's Story, levels 6 through 20 remain locked.
To fix this, simply skip through the last of the currently accessible levels:
1. From the main menu, go to Cranky's Story
2. Scroll to and select Bulking Up
3. Tap level 5 to launch
4.Tap the Pause button in the upper right corner
5. Tap the Skip Level button to proceed to level 6
If you have a similar issue with different levels, try the solution above.