A couple things to look at. The pivot screw in the lever goes into a bushing first, then through a wave washer, then into the body. The lever rides on the bushing. Make sure the screw is tight and not pinching the lever or washer.
Another problem is when powder gets between the expanding sleeve and the die body. It can get packed in there so tight that the sleeve will no longer move freely. Take the powder through expanding die apart and clean it.
Oil is a bad idea as it ends up with powder sticking to it. Powdered graphite is the best lubricant to use. Powdered graphite is available at most hardware stores (often used as a dry lubricant for locks).
