Stunted growth

Geri4

I was given a liquid amber tree as a gift about 5 years ago which does not seem to have grown at all.  It is positioned in a mainly sunny site (shady in the mornings), acid conditions, clay soil, but on a bank so well drained. It looks healthy and produces normal looking leaves every spring which turn their characteristic gorgeous colours in the autumn.  However it is still about the same height as when originally planted and I can't understand why.  It was container grown and as far as I remember had a normal looking rootball when planted.  Any ideas?