Try a clean out and purchase another juice to refill. I'v found the same juice all the time tends to become flat and tasteless.. I use cartomizer cartridges and rotate flavors that way. If you rotate tanks, that can become a little pricey.
For now, you might want to go and brush your teeth and lightly brush tongue with Colgate, Crest , etc.. This helps to refresh your taste buds-works everytime for me.