Fabric puckers

Stitch length is too long for material - Decrease stitch length.
Needle point is blunt - Replace needle or correct size and type.
Incorrect thread tension - Reset thread tension.
Presser foot wrong type - Check you have the correct presser foot on machine.
Fabric is too sheer or soft - Use material backing or tissue paper.
Using two different sizes or kinds of thread - Upper thread and bobbin thread should be the same size and type.