If I remember right, the limit is for all shields together. So, you may be trying to buy 8h ones, but already have your quota met by the 24h ones (or vice versa). Please, check if that is the case. If it is not, please, either provide your ID string (you can get it by clicking on it in the bottom left of the Help window) or specify which platform (Plarium (app included) or FB), server (The Pirate Bay/The Golden Coast/Sirens' Strait), and coordinates you're playing at.

I don't know why this limit. One of weird Plarium's calls, I'm guessing.

linfoster said:

so how does this resolve or fix anything, no one answers, no one cares, so is this how they get players to quit

It resolves the matter by explaining how the game works. In order to be able to buy those other protection shields, you'll have to spend the ones you already have. I'll ask them to reconsider this, but even if they do, it'll take months. So, spending the old shields is the only thing you can do here.