Inline Cable Adjuster

Hi. I have a Giant Propel and yesterday the FD adjustment went out – probably due to cable stretch as its not done many miles. Anyway, the cable has an inline adjuster just before the cable disappears into the frame, so I thought adding a bit more tension with this would sort it (chain rub in higher gears). However, turning the adjuster seems to do nothing – the adjuster doesn’t expand or shrink and the tension remains the same. Is there a specific method to using these or is it knackered? I’ve tried holding the inner parts of the adjuster bits top and bottom, and turning the adjuster but again nothing seems to happen.

Thanks.