The best advice for the most common brand, Selwood Products:
Using a spirit level check the ground boards are level. If the bubble is to the side then check the top level of the play deck is the same. If it is then move the climbing frame to the side and cut grooves where the floor boards would be. Check the playset level by placing it in the grooves with the spirit level and adjust the depth of the grooves accordingly.
Check the main legs, are they concreted? If they are you have a slight issue. If they are not cut around the legs that seem to be higher and drop each one into the small hole to correct the problem. The Plum Products playsets can be heavy so you may need help doing this. They are easier to adjust into slopes in your garden though with just digging a small hole per leg rather than boxed ground boards like Selwood Products sets.