First of all please look at your variants, are they in pink color? When you are typing the name of the variant, do not forget to use comma

Second popular problem is: adding to much useless variants

Correct example here:

Add variant group called Size

Add variants like: M, L, XL, XXL,

Add variant group called Color

Add variants like: Black, White, Yellow

In this example we will have 12 variations in total. Fast to generate. To not have problems with speed i suggest not to have complex products with more than 200 variaitons.

Bad Example (i saw it often):

Add variant group called Yellow

Add variants like: M, L, XL, XXL,

Add variant group called Black

Add variants like: M, L, XL, XXL

Add variant group called White

Add variants like: M, L, XL, XXL

In this bad case we see that seems it's the same, but person didn't combine colors into additional variant group.

Here the variations total is already 125! Generation time increased, page load also!

And it' a simple example. IF you are creating more complex setup - it can just continusly spinning because of timeout