I had a similar problem during my divorce in AZ (also a community property state).
There's lots of factors involved and if attorneys are giving an opinion based on maybe a brief phone conversation, those opinions won't mean squat to the judge.
I even had to file an appeal on my property.
So let's start examining a few things.
What do you mean by "declare" separate property? Do you point at the property and say "I declare that to be my separate property?" (Sorry, got to get a little humor in here.)
What kind of property?
When did you buy it? Before or after the separation?
How did you pay for it?
Why do you think it's separate?
More questions might follow after you've answered those.