I'm pretty new to cocker's as well, but did successfully time a couple recently. Have you checked ravi chopra's cocker infosheet? Great read. Anyways, yes the lug/sear can wear down you may need to lower the lug a bit to keep it from slipping.
If the sear is releasing as early in the pull as you said you might want to try delaying the firing a little bit. See if that helps. Again that would be by adjusting the lug a bit further down. Meaning down from the body of the gun towards the grips frame.