I might recall, that it was an issue on the game itsself (Europa Universalis IV). But they should have fixed it up to now.
A lot of people had the same problem: getting an PU and this country was not helping during a war. I think, that was because it had this "cannot be attacked for 10 years because of peace treaty" - trait.

Of course, in this case it could be, that your relations with wex is so bad, it simply refused to help you in times of war.