The protank gives a pretty good vape at 3.7 volts.(mine has a 2.5ohm coil fitted) It's likely that the silica wick has gotten a bit gunked up with burnt juice and that's probably what your tasting.
One easy fix is to replace the head that screws into the tank base, if you have never changed the head you just take the tank off as if to fill it, then grab the long narrow neck part that sticks up from the base and holding the base too, unscrew it, replace with a fresh head , refill and hey presto fresh set up, and fresh taste.
If you have already changed the head then I would suggest you take the head off and remove the little rubber cover, pull the centre pin out and inspect the silica that passes underneath it.
You will probably find it is a browny black colour, you can lift out the top layer of silica as this just sits on top of the coil and is loose.
You can clean the coil of gunk in a couple of ways, the easiest I find is to do a dry burn. This is where you press the power button for a few seconds with the coil exposed to burn off gunked up juice. You fire it for about 3 seconds then release the button, let it cool down for a couple of seconds then do it again. Repeat this process until the brown/black stuff turns grey.
It has now been turned to ash and can be blown or very gently scraped away. The coils should now glow red when you press the button. They are now clean.
Put everything back together and fill as normal then vape. You will probably get a burnt taste for about 5 minutes but then everything should return to normal and taste fine.
This sounds quite complicated I know, but if you search youtube for dry burning videos all will become clear.
hope this helps you to enjoy your vaping experience a little more.