This may be more information that you ever wanted, but this explains some about VHS tapes.  From http://en.wikipedia.org/.

Tapes can fail for several reasons.

• Tapes can become too sticky to play as a result of an increase in the coefficient of friction as it crosses over the heads and can be effected by heat and humidity in the room.
• They can fail due to a loss in the magnetic signal as a result of a decrease in magnetic remanence or coercivity.
• They can fail because the magnetic coating has failed to adhere to the tape backing.
• They can fail due to irreversible shrinkage of the tape substrate.

  Remanence is the magnetization left behind in a medium after an external magnetic field is removed.

  Coercivity:

  In materials science, the coercivity, also called the coercive field, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Coercivity is usually measured in oersted or ampere/meter units and is denoted H_C.

  When the coercive field of a ferromagnet is large, the material is said to be a hard or permanent magnet. Permanent magnets find application in electric motors, magnetic recording media (e.g. hard drives, floppy disks, or magnetic tape) and magnetic separation. A ferromagnet with a low coercive field is said to be soft and may be used in microwave devices, magnetic shielding, transformers or recording heads.

Magnetic tape degrades by known chemical processes.

• When the kinetics of these processes is fully understood, the degradation mechanisms can be modeled and the life expectancy (LE) of tapes can be estimated. The binder systems used in today's audio and videotapes are generally based on polyester polyurethanes.
• These polymers degrade by a process known as hydrolysis - where the polyester linkage is broken by a reaction with water. One of the by-products of this degradation is organic acids. These organic acids accelerate the rate of hydrolytic decomposition. Furthermore, the acids can attack and degrade the magnetic particles.