Why do I sometimes have sections of tape that do not have joint compound behind it?

The condition you describe, commonly known as "blisters", has two possible causes. First, when loading your taper with the loading pump and gooseneck, ensure that the bucket of joint compound has a sufficient amount of material for the loading pump to fill the taper and that you've properly primed the pump. Insufficient compound or improper priming can result in pockets of air in the compound that fills the automatic taper. While these air pockets won't damge the taper or effect its operation, it will leave sections of tape that have no joint compound behind them. Be sure to maintain enough joint compound in the bucket to keep the bottom of the pump covered and prime the pump to force out any air to avoid this problem. (Watch video)

Another possible cause could be that you're not maintaining constant contact of the Taper drive wheels on the wallboard surface. Constant contact of the drive wheels to the wallboard is necessary to maintain a steady flow of joint compound onto the tape. (Watch video)