Bevel The Ends
Many old springs, not only trailer, are very worn.
Some have lost their ability to hold the weight, and must be re arched, or replaced.
In some cases, the springs are still very usable, but have grooves worn where the end of a leaf has rubbed countless times against the adjacent leaf.
When this happens, the result is similar to having rust or debris between the leaves, the spring can't flex.
The cure is very simple, grind a bevel on the end of each spring leaf that slides against another.
Even if your springs don't have any visible wear, it is still a good idea to do this.