why do our bodies sometimes miss out stages of REM Sleep?

Answer 1

There are times when you haven't hit that level of deep sleep for your body to start processing via REM. That is one reason why even if you go to bed early and you don't hit that level of sleep to where your body does REM for an adequate period of time you will be feeling very tired!

Answer 2

REM, or rapid eye movement, is reached when the mind is at its most relaxed. The bodies entire muscle/nervous system is totally relaxed and the mind has no control over it.
Most people do reach REM, in its own way.
There are no 'stages' of REM, there are just stages of sleep.
To go to sleep the body must relax enough for the mind to slow down.
During the sleeping process the mind can be awoken by almost anything, depending on how deep a sleep the mind is in.
REM is almost always achieved at some stage but for different times.
The art is to try and relax the mind enough to fall asleep.

Answer 3

Have you ever seen an engine skip a stroke? or a computer freeze up for no reason? Well we are basically both of those systems wrapped up into one and a lot more complex... so any little thing could cause who knows what. I could give you the typical response of maybe it's stress or anxiety or too much caffeine... but the simple fact is even an educated guess is just a guess.