Rip Tripper says when e-juice goes bad, it loses flavor and viscosity. He said he had two bottles of the exact same juice - one he kept in a cool dark area, and the other he set on the window sill. The juice by the window went bad in less than a month and a half. It also got almost black in color.
Of course it matters what kind of juice, but the safe bet is your juice is old.
Always store your juices at room temp or cooler, preferably in the dark. If you bought a bad bottle, just contact the manufacturer and get a replacement. (LOL unless it's High Desert Vapes - they don't like their customers and won't refund or replace merchandise under any circumstances.)