Samsung dishwasher dispenser doesn't open until partway through rinse cycle
It would be helpful to know if this is a new problem or a new unit. The first thing that comes to mind is "Are you sure you are putting the dish washing soap in the right dispenser"? 

I looked up the manual to your particular model here: 

Samsung Manual

You can see on page 18 and 19 it differentiates between the two dispensers. The behavior your are describing is what I would expect from the rinse aid dispenser.

I also once had issue with dishes blocking the latch from opening completely. For instance, placing a sheet pan parallel to the door. The door would open slightly and the dishwasher tab would not dissolve properly until the rinse cycle.