Which nepali fonts have u installed and what articles r u copying? Ascii based Nepali fonts such as preeti etc. that used to be used earlier are different from unicode based fonts. If you've Nepali unicode installed in your computer and u're copying articles in devanagari from web pages (most of which use unicode based text nowadays), then u should have no problem. But if the article is in some ascii based font, then please check whether u've that particular font installed.

I have preeti, gauri, kantipur fonts on my PC. Was trying to copy paste from Kantipur actually and didn't work,so. Thanks for the suggestion though.

I tried to copy from kantipur and I was sucessful. The font was Mangal. May be you can try to install the font.

Nepaliraja,
U r right that I needed Mangal font in my PC. Now that I have it installed, it is working just fine. Thanks for the suggestion, dude.