There are several reasons, which could be causing it - so, please read on before you fire your digitizer!

= Improper Hooping
 If the fabric hooped too tightly, it stretches the fabric and after embroidering when the fabric is removed  from the hoop, fabric tries return to original state - and in doing so faces a resisting pull and push effect from the embroidery resulting in curling up, try to loosely hoop the fabric.

I would also like to mention here, that sometimes if the grain of the fabric is not hooped perpendicular to the hoop, can also result in uneven or wrapping effect.

= Improper Backing
Backing plays a crucial role in stabilizing the fabric when its been embroidered, so check if you are using a the correct stabilizer - to read more about stablizers click here.

= Improper Underlay or density.
If the design is complex with too many overlapping objects and a high stitch count, then a very dense underlay can also cause puckering - so, reducing the density of both the underlay and the design may help - you can also try changing the stitch angle and the sewing direction.