Why do some videotapes last whiles others don't?

Factual Questions
1

I have some videotapes in my collection that are more than 20 years old. I was watching one last night. Had excellent color. Played just fine. I have others a decade old that are the same way. I have others, just two or three years old, that are a mess – the tape is skippy, the image flickers so badly that you pretty much can’t watch them.

The ‘badness’ of this sort seems more common on the more recent tapes.

I don’t do anything special to store the tapes … I keep them in stacks in bookcases. I don’t buy any kind of special tapes – name brand tapes of the sort that come in bunches of 3 or 5 or 10 at Target. (I bought a couple of cheap, off-brand tapes that were crappy right out fo the box, which soured me on that practice.)

Anybody have any idea why some tapes last and others don’t. And BTW, the ones that tend to last are the ones that have been watched a fair amount over time.

2

Is there any relationship between the bad tapes and the playing length of the tape. I ask because the reason VHS outsold Beta was because the amount of playing time on a single tape. The (newer and) longer playing tapes were also thinner.

Another factor could be manufacturing plant (not name brand.)

Also, the newer tapes would of been recorded with older (and dirtier head) equipment.

3

What Can Go Wrong with Magnetic Media?