Ficus Won't Stand

[Howard Schlossberg]

I have a ficus outdoors, planted about two feet from the house, and 
currently standing about 12-15 feet.  It has always seemed very healthy, 
but a couple of weeks ago I went outside and found it leaning over, its 
base slightly pulled out of the ground from the weight.  I tied it 
upright to the house, but the leaves are turning yellow.  Another nearby 
ficus is still very green.  I'm in Los Angeles, so the temperatures here 
have been moderate.  Should I try digging a deeper hole for it, or do 
you have any other suggestions?