Try putting the old return spring back in and using the mag that came with the gun and see what happens. You may have got to heavy of a return spring for the gun and it is not cycling all the way to the rear when fired. I guess you have run more than 2,500rds through it,right? Good luck.

My first suspicion is recoil spring.

I have a 16 lb recoil spring. It is stiffer than the one I removed, it was old. If I had put in an 18 lb spring then I would have immediately figured that was the problem. I used the three mags that I own, two brand new and the original that came with the gun. The original loads a lot easier than the two Chip McCormick mags so I figure the original mag is getting weak. Could I have under-lubed the gun?

Lubing will not be a big factor in your problem. Did you try the old slide (recoil spring) spring? I don't think it is your mag springs, if they were working before they should work now.

Is it every round, or just the last round?

I was at the range today, testing a friend's LW Commander. It had a nose-up failure to feed with every 8-round mag I tried (Kimber, Wilson, McCormick), but only on the last round. It did not have that problem with any of the 7-round mags I tried. I don't know; maybe there are just some guns that don't like 8-rounders?

On yours, I suppose you could put the old spring back in (if you still have it), just to rule out the new spring as the problem.

When they will not feed the last round it is a mag spring 98% of the time or a bent lip on the mag from dropping them like in competition. My 8 and 10rd mags have never failed to feed and if they did I would get new springs.

I think you might have a problem with the McCormick mags. I have a 45 springer and the two seven round factory magazines work fine I bought two McCormick 8 round mags and they jam a lot especially with fmj