There are lots things that could cause that but without more info its hard to say for sure.
Are they Edelbrock heads? If so new or used
Crane shafts have a right and a wrong way for install for proper oiling. Harlan Sharp can go on either way.
Like asked above were the shafts banana grooved?
If so were they done right. Some guys make the grooves to long and the oil sneaks out past the rocker.
Did you limit the oil going to the head. Some guys do.
Were the rockers shimmed right side to side?
Were the rockers checked for proper sweep on the valve tip.
How many runs did the rockers have on them?

I personally never use those springs as they do not limit rocker placement properly. I usually take 3-4 hours setting up rockers the right way.