Growing those trees in clay ground can be done, but it's hard and requires a lot of work, amending the ground yearly by burying the old organic mulch in, adding horse manure and perhaps do some backbreaking drainage work.
You have my sympathy because the soil here is basically clay mixed with a lot of rocks: the only advantage is that being on a hillside I have some drainage but I have lost count how many plants I've lost or had to pull because they were becoming awful despite all the extra care.
