We don't see our heroes "not doing anything" about the shield thing. We don't see our heroes, period. For all we know, LaForge could have been working his fingers to bleeding stubs trying to punch in new shield commands, all of them voided by the VISOR tap. The camera just didn't go there.
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As for scanning for Soran's VISOR tap, why should our heroes have been successful? Soran concealed the device. We could give him credit for doing it right - it took Data several days to figure out how Tal'Shiar did a similar thing in "Mind's Eye"!
Timo Saloniemi
Geordi comes back from captivity, and as soon as he's returned to engineering, the Klingons start blasting holes in the ship. No one saw the connection? He should have thought, "Hang on a minute, something's not right here", put a bag over his head and made Farrell remodulate the shields.
Maybe that incident was the last staw. After years of being blinded or used by bad guys, he was finally convinced to ditch the VISOR and go for the Terminator eyes.