Catalog Price Rule doesn’t work on a group of products

I have Magento ver. 2.2.5,
and I created a simple -20% discount rule on all products with the Catalog Price Rule. But the problem is, that the rule doesn’t work on some of the products and I have no idea why.
Could anyone please help me out? I have tried everything I could but can’t figure out what the problem is..

some info about the shop:
I have 2 customer groups and they have different prices for the same assortment.
(One of them can purchase the products at a “normal” price, while the other group, in which I have special key customers, can purchase all the products at a lower price. I created these special lower prices with advanced pricing, and these lower prices are displayed on the front end as it was a special sale – so the key customer can always see the normal price struck through and the reduced price next to it.)

Moreover, I have simple products and configurable products too.

The problem is that the rule does not reduce the prices which my key customers see, only those, which are for the “simple” customers.
However, if I make a cart price rule, the reduction applies to all the prices. I doesn’t matter which product it is or which group the customer belongs to.

It would be great if someone could help me,

thanks a lot!!