Welcome! You will find lots of answers here:

You may not be using enough or the right weight of stablizer. Your hooping may not be tight enough. Also, I believe that if you stretch the fabic too much when hooping that can happen.

If you're doing a fill-type of stitching, your density may be too high. Don in TX

Thank you for asking. I don't like the puckers either. It's one reason I hesitate to embroider for payment. I have a lot to learn, too!

Stabilizer is probably the problem. If you will spray starch well, and I mean really stiff, it will help a lot. Then using a tearaway stabilizer on a woven fabric should give you better results.
Overly dense designs will cause puckering too, sometimes regardless of how well you stabilize.

the lady who sold us baby loc said to use a no-show mesh fusible and maybe a tearaway under hoop