According to Monrovia, 'Baby Cakes™ is an exciting, new, thornless dwarf blackberry that presents a fireworks-like display of large, juicy berries in the summer! Its compact habit makes it perfect for patio pots. In most regions, this blackberry will produce twice in one season. Produces on second-year canes.'
It says in most regions it produce twice in one season, but not in all regions. It also says it produces on second-year canes. Your primocanes were created this summer, so should not expect them to bare fruit until they are in their second.
Working as a nurseryman for many years, I have often found that when new varieties come out, they sell them as the most amazing berries to ever come on the market, but when it comes to the home garden, we discover otherwise. You have to remember the growers have ideal growing conditions, where they are able to get these plants to produce in ways that you and I will never be able to reproduce.
Monrovia has three locations in the United States. The two biggest are in Oregon in a region known for producing amazing edible and ornamental plants for the nursery trade. The largest growing operation is in California, where they have some of the most prime weather for growing amazing plants. Bountiful sunlight, warmth and very long seasons.
I think it is one of these two things or a little of both;
Maybe it needed time to settle in, before it starts to show you ever-bearing characteristics.
The grower is able to produce what only can be produced under the most ideal conditions. Ideal conditions start with knowing the exact chemical make-up of their soil, knowing when and how exactly to fertilise this particular plant. Not just blackberries, but this variety of blackberry. You will not have some expensive equipment to read data and laboratories to analyse your growing conditions.
In the end you will have fruit that you grew yourself. You can enjoy it knowing you did that.