Some commercial tapes have copy protection on them, to deliberately prevent anyone from duplicating the tape. Essentially, the recording has some deviations from a pure, standard, video signal, which are propagated through to the recording device by the VCR which is playing the tape. These deviations are usually enough to prevent the recording device from getting a good copy. Sometimes you might lose "vertical hold", or the colours cycle from bright to dark, when you play the copy. A TV ignores all the copy protection signals, and shows the picture as normal if it's getting the signal straight from the player. I have heard of people who have had problems with their home theatre projectors, which couldn't even show an original copy of a copy-protected VHS/DVD recording.
I had a very early PAL video recorder which was "immune" to copy protection. (We bought it in 1980, I think, to record "Life on Earth" and "Cosmos".) I've always suspected that there was a conspiracy to add support for copy protection into all new video recorders later in the 80's.
MACROVISION was the/a pioneering force behind copy protection in video tapes, and you'll probably see their name in the fine print on the back of many of your DVDs today. DVDs are typically copy-protected, too -- If it says so on the cover, you probably couldn't play them on a DVD player and record them on your VCR with any more success than you're having with the video tapes. But, cheap Chinese DVD players often feature "macrovision removal" which means that they won't pass the evil signals through.
As a resident of Australia, I'm in a PAL region, not NTSC. But over here, we used to be able to buy little boxes which you could put between the player and the recorder to filter out the copy protection signals. I.E.
player ---cable---> "video stabiliser" ---cable---> recorder
When our 1980-vintage VCR eventually died, I made my own "video stabiliser" from a kit from Jaycar. Nowadays, I think you can still get PAL "video stabilisers" pre-built for much less than $100 AUD.
I would suggest that you google for "macrovision remover" or "video stabiliser", and you may find a tool which will help you. Make sure it's designed for your country's video standard. (NTSC for the U.S.A.)
Or, for more background information, search wikipedia for macrovision. The article may not sound too promising for many of you: The Digital Millenium Copyright Act may have outlawed the sale of the type of device I'm talking about in the U.S.A. Maybe you might still have luck finding one, if the device fulfils another purpose besides stripping off the copy protection signals. Some vendors might still be selling them with the defense that they also amplify the video signal. You might be able to D.I.Y. like I did, but you'd want previous experience with a soldering iron. (And not my elder brother's type of "experience", picking one up by the wrong end... Don't try that at home.)
Tyler.