Hi and welcome to Just answer.
Initially, if you have problem on underneath. The cause is you upper tension maybe loose or tight. Try to clean you upper tension disc using scrap fabric, just move forward/back in between of the disc. Also try to adjust the tension until you got the right tension for your stitches.
Hope this information will be helpful.
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Hi. Thanks for your attempt to help me but your answer did not resolve my issue. Since I was referring to two new machines and I was asking about the thread having oil/grease on it. I needed to know if that was causing irregular bottom stitches. Your response was very basic and I do not want to pay $39.00. I am willing to pay $15.00 for your answer. Please credit my credit card to reflect a total charge of $15.00 and not $39.00. Thanks.