You are probably not holding the mandrel completely horizontal. Try putting a mirror in front of you to check this - it's handy to see who is creeping up on you as well! If you see that you need to work on one end to correct it, just heat that side of the bead and dip the mandrel down at an angle to let gravity move glass that way. You can also roll the centre of the bead on your marver to move glass to where you want it. Um, you can also put an extra ring of glass at the offending side. Lastly, you can do the tuning fork trick - heat the bead to soften, and then tap the mandrel vertically on the bench. (You can see I've had the same problem as you when was starting out!!
PS Sally Carver has an ace set of dvd's you might like to get
PPS Cross posted with Spexy and Kay! We all want to help you