Whether you intended to move or not, you may have moved more than the shallow DoF would permit. You cannot stand perfectly still unless you're a corpse. As an exercise, set your shutter to 5 seconds and go take a picture of anything; if it's tack sharp I'll be impressed, but it won't be because you moved even if you were trying to stand still.

There's three ways you can move the camera, left/right, up/down, or forward/backward. As a human being you cannot stand perfectly still no matter how hard you try. Shutter speed fixes the left/right/up/down movement, but won't fix a shallow depth of field issue. Up/down/left/right all stays within the depth of field, forward/backward may not if your DoF is too small.

You move, and by extension your camera does as well. In normal situations the most noticable movement is the left/right movement that causes the subject to move across the sensor. In normal situations you don't notice the front/back movement because there's enough DoF that your subject stays within it. However, at macro distances the DoF is comparatively very shallow so things change and tolerances become very tight.

@Morpho in the same way that your body moves slightly from side to side you also move forward and backward (especially as you breathe). So if your DoF is very small that forward/backward movement may cause your subject to be out of focus.

Women typically want smoother (younger) looking skin so making her pores look as dramatic as the old man's wrinkles would probably not make her happy; soft lighting (which has the opposite effect) is generally preferred, but it's still very flat and not visually interesting. That lens isn't bad, but they make them better (as in, more expensive); knowing the limits of your equipment and how to work within them are key. And yes, centering everything is generally a no-no. It can be done, but generally isn't desirable (see: the rule of thirds).

If you shoot wide open all of the time you may lose detail as the depth of field may be too shallow. Sometimes it's appropriate to shoot at F/4 or F/8 to make sure everything is sharp (if that's the goal). For example, I believe Joel Grimes shoots everything at F/8 for his composites. It's a stylistic thing, however, and not a hard and fast rule.

It's from Vanguard and came attached to a ball head that is now discontinued. The flaw with this particular one is the adjustment screw is prone to moving when not clamped shut which means the clamp may be too tight/loose when you go to use it again or may fall apart altogether.