What do you mean 'it won't sew right?' What does it do? Are you using the same thread in the bobbin? Have you cleaned your machine? If it is leaving loops on the bottom of the fabric it is tension. Clean your tension disks and rethread your machine before adjusting the tension. If the thread is breaking it could be the needle. There are so many things that make a difference. Poly thread should not be a problem. I often use it.