Sure, you just need to get away from it for a while and not think about it, just go for a drive if you can, or a bike ride, bring a notepad in case something comes to you, but just do something completely different than writing
I get that all the time. Generally I write out the summary or bullet points for the plot, write up all the characters. Then I probably manage to write one chapter and I get writers block lol. Mostly because I want to get to the main juicy parts of the story lol. And sometimes the whole first part of the story goes well and I get to the part I finally waited for and I don't know wtf to write lol. Usually I go and watch some movies and get a bit of inspiration.
I think it's normal to have some problems at that point, because until now you had an idea of general characteristics and physical appearance, but when it comes to explicitly describing how they react to situations you haven't had a chance to explore that yet. Try putting them in interesting or challenging situations that help confirm OR challenge the personalities as you have laid out for them. Maybe they surprise the reader AND you with how some characters can evolve.