I am using a plastic worm for catching bass, on a texas rig set-up. I use a throw and reel in technique. I usually go fishing around 6-8pm, but i am catching nothing >... I wonder what im doing wrong...?

p.s. i know that the lake has bass :-/

http://www.bassresource.com/fish/plasticworms.html

instead of reeling it in, let it sink to the bottom. wait a few seconds and give it little hops. fish it slow.

At that time of day in the summer, depending on the lake, the bass may not be interested on a lure on the bottem.  Use a lighter weight and pull your rod tip up high to get it up there a bit.  That may help a bit.  

If you've caught bass out of the lake before, you may want to consider matching your worms to be about the same size as other lures that work.  Sometimes a 7inch worm is just too big.  Scaling down may help out some too.  As far as colors, if it's watermelonseed or watermelon red, you're doing something right in my book

Slow down and drag it on the bottom/hop it etc.....