On Wednesday, June 24, 2015 at 12:39:49 AM UTC-4, Mike Spencer wrote:
Two years now, most broccoli plants have headed out very early, giving
2" to 4" heads. In the past, with same or similar soil and
manure, we've flourishing plants that yield 12" to 14" heads.

Climate in NS is a bit wetter and usually a bit warmer than western
Massachusetts or southern NH. We've had an unusually chilly and wet
spring this year (after the seedlings were in) but not last year.

Any suggestions to remedy this? Identify the likely cause?

--
Mike Spencer Nova Scotia, Canada

Perhaps your soil has gotten low on boron. Boron is a micronutrient that is necessary for good cole crops. The addition of a little laundry additive (Twenty Mule Team Borax) will solve the problem is this is the cause.
Steve