thinggies

Look, they probably DID have them then, just you are being rather literal about only thinking they are on when you can see them. Try not to over-analyze!

course you can see them when they're on....they're blue shinny thinggies..

Sigh... but on the assumption that they do not randomly turn them off and we see them plenty of times WITHOUT the blue shiny thing we can assume that the shields are not always visible! Whether that is deliberate or accidental on the part of ILM, I wouldn't read too much into it.