Possibly you have some cartomizers which are on the old side. Maybe an older batch.
I'm thinking you have a KR808d model e-cig. The official forum for that model is
here. You can find all kinds of tips and tricks, plus a listing of suppliers, where to get reasonably priced cartomizers, etc.
The plastic taste might mean that the cartomizer is running low on
juice. Do you have some
juice to refill it with? You can find lots of refilling tips on the subforum linked above. From dripping a couple drops into the battery end to popping off the white cap, tipping at a 45 degree angle and dripping into that end, to the "spin" method of of SERIOUSLY replenishing the cart.
Hopefully you have an extra battery on hand too.
Try another cart, or refill the one you have, if adjusting the fit between cart and battery doesn't help.
Good luck with your KR808. It's a great smoke!
Might be just a matter of screwing the battery on a little tighter or backing it down a little.