most all micros, can be locked up, with a Ph imbalance, the exception being molybdenum, brassicas need boron manganese molybdenum and iron.
As for what could be effecting the Ph. I would suspect soil drainage, beings as it was stated to be clay soil.
even with amendments (our soil is high in clay, with a organic content of over 6%) typically, clay soil is alkaline (has a high pH)
and even though it has been amended, clay particles, after years of watering/rain and foot/machine traffic...will settle and bind together, effecting drainage, maybe not within the top 6-8 inches, but beyond that, in the root zone of most deep rooted crops, it can get very heavy