How do I fix puckering when sewing seams?

Your tension is almost definitely the culprit - most puckering happens when it's too tight, so try loosening your top tension dial by a few numbers and test on scrap fabric first! Also make sure you're using a fresh needle (they get duller than people realize) and that it's inserted correctly, since a bent or backward needle causes all kinds of weird stitching issues. Cotton can be finicky, especially with polyester thread, so some folks have better luck switching to cotton thread instead - it just plays nicer with natural fabrics! One more thing: are you using a presser foot pressure that's appropriate for lighter fabrics? If it's crushing your cotton, that contributes to puckering too, so check your machine's manual for the right setting.

Thread quality matters way more than people realize - polyester can be brittle and inconsistent, so switching to a decent all-purpose cotton or cotton-wrapped polyester thread often solves puckering that tension adjustments won't fix. One thing to watch: if you're using old thread that's been sitting around, it gets weak and tangles inside your bobbin area, which creates the same puckering effect but no amount of dial tweaking will help.

Puckering on a Singer usually isn't just one thing - tension and thread both matter, but the one thing nobody seems to mention is your presser foot pressure, which can bunch up fabric if it's cranked too high, especially on lighter cottons. Try dialing it back a notch or two and see if that smooths things out alongside whatever you adjust with tension.

Have you checked whether your machine's been threaded correctly? I had the same issue with my Singer last year on a linen project and drove myself crazy adjusting tension before realizing the top thread wasn't seated properly in the tension discs. Once I rethreaded it (with the presser foot UP so the discs actually open), the puckering stopped immediately. The other answers here are solid - tension and thread quality definitely matter - but honestly threading is the first thing to verify since it's so easy to miss, especially on older machines. If that's fine, then yeah, loosen your top tension gradually and test on scraps, and if polyester keeps giving you grief, cotton thread usually behaves way better on cotton fabric anyway.

the biggest thing i'd add is that cotton thread actually plays really nice with cotton fabric in ways polyester often doesn't. polyester can create friction issues on naturel fabrics, especially if your machine's been running for a while and the tension discs have any lint buildup. before you go swapping thread, try running a thin needle threader through those tension discs - sounds weird but lint gets trapped there and throws everything off. i've seen folks waste hours adjusting settings when the real problem was just dust.

one thing nobody's mentioning though: are you using the right needle size and type? a dull needle or one that's slightly bent will absolutely cause puckering, and a lot of people don't realize needles wear out faster than they think, especially on cotton. if you've been on this dress for a while use a fresh needle - they're cheap. also make sure it's inserted all the way up into the machine, because even being slightly loose creates tension issues that feel like they're coming from your dial settings.

the presser foot pressure thing is real (somebody mentioned it) but also check if you're pulling the fabric through. i know it's tempting to guide it but the feed dogs are doing the work - just let it flow. with cotton on a basic Singer you want medium tension, not loose and not tight. do a test seam on scrap cotton first, then adjust in small increments. if you're still getting puckers after a fresh needle and checking those tension discs, then yeah the polyester thread swap makes sense, but my money's on one of those first two things.