Can't shift down - no resistance when trying to do so

I'm having a bit of a problem and as a newbie to bike maintenance, I don't know where to start.

Today, as I was trying to shift to an easier gear on the back, I was met with no resistance from the lever. The lever to shift to a harder gear works just fine but the other one does absolutely nothing, at any gear.

This seems like an easy fix but as I said, I'm pretty clueless. I don't know what to search for because I don't even really know what my problem is. I have Ultegra components if this helps.

(My first post here, I'll make a proper introduction later)

I was having this problem intermittently with an older ultegra shifter. The guys at the shop sprayed some lube inside and everything has been working for now..... (they did say it was wearing out though)