You can probably get away with just adding a BB ahead of the ejector spring to give it a little more force, you can order the spring from H&R or Brownell's, or get one from your local hardware store and cut to length.
But it probably won't solve the problem caused by neck sizing only, once the brass grows too long, pushing the shoulder back is the only way to fix it using the same brass. The internal flex of the action allows the brass to grow each time it's shot, eventually it's too long for the chamber, you can easily see this by trying to chamber a fired case, it should be flush with the chamber face, if it isn't, it needs to be full length sized.
Tim