Protein loss from the kidneys is called albumin-urea, basically it indicates that the kidneys are not functioning properly and are not able to retain the proteins
inside and are excreting the substances (proteins) which it should retain.
There are many reasons of this condition , stating from entirely a normal phenomenon for some one with very small amount to a fairly advanced kidney disease in cases of huge protein loss.
In case of post transplant patient, rejection ( although it has been 6 years) to different kinds of glomerulonephritis
can be the causative agents.
I think biopsy
is the the way to go to see what exactly is the problem so that it can be fixed accordingly.
THANKS and best of luck.