This was happening to my 505 stroker. I was using a Comp Cams hydraulic set up. Was very frustrating and usually at start up. Nothing worse than having to pull a valve cover in parking lot to get the pushrod back in. My engine builder finally fixed it. I'd have to call to see what specifically he did if you still need help with it. I believe he used a good amount of pre load and some different springs/retaining clips. Have not had an issue since he did that. I've known a few people with 440's locally that have had issues with similar set ups.