I am not a bonsai expert so please forgive any stupid comments I make!

I have 1987 Juniper Bonsai that has great sentimental meaning, I also
had another much young Juniper Bonsai - both of which seem to be
suffering from some sort of definciency - the leaves(needles?) on the
younger started to die and eventually the tree died. My second, much
older tree has now started to turn brown too and I'm not sure why,
here some points that might help:

1) Other bansai (not Juniper) are still thriving
2) The dying Juniper has been in the same location for over 5 years
and was fine until very recently
3) They are regularly feed (not too often tough) with plant foods
4) They are watered regularly.

Are there some obvious or basic things I should check, particular
needs of a Juniper, common diseases or mineral deficiencies?

Any help would be greatly appreciated!

Troy.