Some vertices not attached to armature

Hi I am a newbie. I have been going through the tutorials, and I think I am doing something wrong. I add a cylinder, loop cut it in the middle, then add an armature. I make sure the envelope covers the area.

The problem is that sometimes it is as though not all of the vertices are connected to the armature. When I rotate one of the armatures, most of the cylinder rotates correctly, but a couple of the vertices do not rotate, and stretch instead.

Can anyone give me a pointer ?

Thanks in advance,
lactose

you could use weight paint or vertex groups to help the envelopes out (i prefer weight paint)

here is a link to pretty good series

Thanks. It dawned on me that maybe I should check the armature envelopes from the side view also. I went back and re-did them and now they work properly so maybe that was it.