One reason image scenarios aren't created is multiple selections are enabled. Product Builder isn't able to show two preview images at once, and image scenarios will be disabled. Another possibility is your builder setup has reached the 10000 image scenario limit. In the latter situation, you could reduce the number of image scenarios being created by using conditions.
Step 1: 6 choices
Step 2: 6 choices
6 x 6 = 36 possible selection combinations
If you add step 3 with another 6 selections
6 x 6 x 6 = 216 different image scenarios
As you add more and more steps the number of image scenarios will sky rocket.
If you set up conditions where step 3 will only appear when step 2 selection 1 is selected. There will be the original 36 image scenarios from the first two steps, and only an additional 6 image scenarios when step 2 selection 1 is made. Product Builder will know that step 3 will only be used some of the time and will not include all of the extra image scenarios because they are not relevant. When you have 15 plus steps and your using conditions to only conditionally show steps you can save yourself 1000's of image scenarios.
Visit Add Conditions to Your Product Builder for help limiting your image scenarios.