Sugar...it could me a multitude of things...all at once even..
First, good fabric is a must!
Second, wash and iron that fabric before you cut it.
Third, always use the same ruler for cutting out the entire project. ANd make sure you have fabric gripping dots on the bottom of them!
Fourth, make sure your seam allowance is correct...mark your machine base if you have to, or put some Mole Skin down to help guide you....I have a 1/4" foot and can't live without it...
It takes practice to get all of these things done right, all of the time...but you will get it...