I don't know exactly what you're doing, but you should always make sure you unzip (unpack or whatever) the folder that contains the plug-in, check the directions. Some require that you put the entire folder in the wp-content \ plug-ins folder. Others require that only the file inside be put there. I can't tell you which is which, but I will say this: There has never been a plug-in that wouldnt install.
On the other hand, some require that you manipulate the theme files with some code changes, and some, frankly, don't work at all, or create problems.
Understand that I'm not a WordPress expert. I learned a few things after my Webmaster took an extended hiatus, and I found it wasn't so painful. In fact, I constantly experiment with plug-ins to enhance the look of my commentary site, http://macnightowl.com for better or worse. I learned fast how to get out of trouble. In short, I probably know enough to be dangerous, and certainly enough (as little as it is) not to be afraid of it.
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