Often, especially in password fields, VoiceOver will think that the VO keys are locked. Pressing tab from a password field, for instance, will often generate the response, "Ignoring next keypress," and trying to paste into the field will fail with the message "next visited link not found". We have unfortunately not yet discovered a consistent way of working around this once it starts, short of restarting VoiceOver or hitting the VoiceOver modifier keys several times in a row.
My solution is: VO-arrow away from the secure edit field. Then, toggle the caps lock key (i.e. turn it on and then off). VO-arrow back to the secure edit field and press VO-shift-down arrow and type your password. I've noticed this problem in the App store and some other areas, and this seems to work for me.