I'll be honest: a wobbly bench is almost always caused by wood shrinking as it dries, fasteners loosening through weather changes, or one leg losing contact with the ground. You can usually fix it in an afternoon and stop it from coming back.
Start by flipping the bench over or placing it on a flat surface. Rock it gently to find where the movement is coming from. If you see gaps at the joints, the wood has dried and pulled away from the screws or tenons. If the bolts spin without catching, they have worked loose. If one leg floats, the frame has twisted slightly or the ground underneath has settled.
For loose bolts and screws, remove them and add a small amount of exterior wood glue into the holes before driving them back in. Even better, replace old hardware with stainless steel or coated deck screws. Use flat washers and lock washers wherever you can. If a screw hole is stripped, pack it with wood glue and toothpicks or a small dowel, let it dry, drill a pilot hole, and reinstall the screw.
For joints that wiggle, add metal corner braces or L-brackets where the seat meets the legs. Use exterior-rated screws so they do not rust. If the bench uses mortise-and-tenon joints, carefully tap them apart, clean off old glue, re-glue with waterproof wood glue or polyurethane glue, and clamp overnight.
If the bench rocks because the ground is uneven, do not keep over-tightening the frame to force it level. That can twist the wood and make it worse. Add adjustable feet or use small composite shims under the low leg. On soil, place the legs on pavers or crushed stone so they do not sink unevenly.
Prevention is the part most people skip. Twice a year, snug every bolt, screw, and nut. Apply a penetrating wood sealer or exterior finish before the rainy season and before winter. If the bench sits on dirt, lift it onto bricks or pavers during long wet periods. When buying or building, choose cedar, teak, or pressure-treated wood and insist on stainless or galvanized hardware from the beginning.
One extra trick: after everything is tight, brush a little exterior wood glue over any joints that cannot be braced, then wipe away the excess. It adds quiet reinforcement without changing the look. If the bench still wobbles, check for small cracks near the bolt holes because a split can let the fastener move no matter how hard you tighten it.
Realistically, an outdoor bench only needs a 15-minute checkup twice a year. Do that, and the wobble should stay gone.