If starships can raise and lower their shields at will, how do they fail in battle?

The question is this: Starship's can materialize their shields instantly, when there were no shields (or no more than navigational deflectors) to begin with. However, when there were some shields, and they take a hit in battle, they do not rapidly rematerialize as if they were being brought up for the first time. What is the difference between "shields up" and "repair the shields"?

I know generators can be damaged, but we might surmise that regenerating slowly after taking a hit is a property of the shield itself.

Captain Picard, amongst many other Captains, can order shields up after they haven't been activated for days, weeks or even months. In the heat of battle, it is then the Captain's option to lower them completely, whether for transport or in certain most dire situations to save power. However, when he raises them again, they won't be any stronger than when he dropped them.

So: how is it he was something that can materialize instantly, but cannnot rematerialize instantly?