Cannot get plugins working

Hi,
I’m having here a strange struggle. I have overtaken a project (relatively simple) form other person. He has build a page over commercial sold theme. 1st thing is that that theme by default camed with some shortcodes, and plugins which I cannot see anymore. 2nd no matter what kind of plugin I install and activate they simply do not work. Mostly every install as it should – meaning all the buttons or menus are created, sometimes it doesn’t. Even those that made their buttons for editing post/page are present and they are injecting/creating shortcode it does not bring/produce any effect on the page. I was searching sth like that on the Net, but simply did not get a single case as mine. Am I missing sth simple or nasty code left by the other developer?