There are so many applications on Ubuntu Apps website which when i try to open with Ubuntu software center, they are no where to be found!
Does it help if you select Software Sources from the edit menu and make sure that the (universe) and the (multiverse) options are checked?
Once it is done make sure to run:
sudo apt-get update
Also in the software center technical items are usually hidden. You can display these hidden items by clicking the link in the lower left hand corner (after you have issued a search).