Do you get an error message? Or does the query simply return no results?
There is a dgraph flag that controls the maximum number of terms to evaluate when doing a search called search_max. This value defaults to 10 which makes me think it might be related to your issue. In a standard search mode, I thought it ignored everything beginning with the 11th word but Boolean may behave differently.
If you try adding a --search_max 20 to your dgraph and test again, I'd be curious what your results are.