My advice would be to debug both microflows and see if the data is correct: eg, if the Cart Ids match. 

When is the second microflow being triggered?

I would also move the Cart retrieve on the first microflow before the IF and then use the result in the IF condition. Also, the commit at the end is not required since you are already commiting in the Change activity. This is not related to the issue, it's just some improvement tips.

Let me know if this helps.

Because when you create the cart, you do not associate it to the Customer, but only to the Product:

Therefor when doing the retrieve Cart by Cart_Customer, you do not get a cart. So the solution is:

in the activity Create Cart, also associate it to the Customer

Here is an images of what I did

Hi, have you figured out how to do it?

I'm struggling with this part too