Couldn't they have recreated the shield after it was destroyed or reinforced it when it was in the process of being destroyed?
1 Answer
They didn't have time.
If you recall, the creation of the shield took several moments. It was a combined effort of the teachers and it slowly grew to encircle Hogwarts.
When the shield came down, Voldemort's forces immediately rushed onto the grounds of Hogwarts from several directions.
At that point, the teachers were ocupada with defending the castle.
Conjecture
Probably the shield is super vulnerable when it is being constructed, so trying to hastily make a new one would have been wasted effort.
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"combined effort of the teachers"? I think you mean "combined effort of the teachers and Molly Weasley". Because, movies.– ibidMar 30, 2017 at 0:52