@rakkie
What were the actual 5 preferences (as in, types of school) that you ranked on your CAF?
How many grammar schools did you name?
Do you mean that the school that you were allocated a school not named on your CAF at all? Or just a non-selective school that you did actually apply for, just ranked lower on your CAF than all the grammar schools?
On March 1st, your LA, along with the information that your DC had been allocated a place at School X, should also have told you why you had not been offered a place at the schools you had ranked above School X (which may have been all of them, if none of your preferences could be met. Depending on your method of application (paper form, or online?) this would be in the form of a letter, or an email and / or details on the LA's admissions portal, which you can log in to.
Forgive me for asking, but you didn't actually only need one grammar school, with the assumption that if your DC didn't get that one, they would be allocated an alternative grammar school (rather than either a lower ranked non-selective school that you actually listed, or the nearest school with places available, of none of your preferences could be met)? Is this actually what your LA says it will do?