You're referring to a burn through. It means that you are trying too hard to "consume your tack", and are basically burning through an existing weld. All you want to do is tie in good to both sides of your tack. Probably a little less grinding near the center of your tack would help. If you grind too much on your root, you are opening up the possibility of a burn through. A little less grinding, and more heat, maybe..............hope that helps.
As for making the tie in to the tack try feathering the tack a little deeper to allow you to reconsume the edge of the tack as you travel onto it, also once the keyhole closes add a little pause and build up a little more metal in the tie in area. Suck back is caused by traveling too quickly or not adding enough metal. When it comes to installing consistent roots only practice and experience will get you there. Try installing your tacks at 3o'clock instead of from the top to prevent heavy push through and don't get in a hurry welding off of or onto your tack. Good luck.