Check for coil bind or maybe the collar/keepers are hitting the guide/seal. Put a good pushrod in and push down on the valve or rocker with the valve fully open. You should have some movement left before it hits or binds.
Plenty of clearance in guide/stems?
I always use moly assy lube on valve stems.
Are you positive the guides haven't moved? You might can try grinding a little off the top of the guide.