I'd check to see if you have a drainage issue, gophers eating the roots, or maybe you have drip irrigation and the second tree is not getting any water. If the soil is staying too wet, you could be rotting out the roots. If it is gophers, you will see some obvious signs of tunnels and mounds. If an irrigation problem, the area at this tree will obviously be quite dry.

If none of the above, could this spot have chemical contamination from paint, paint thinner, herbicides or building construction rubble? You might want to dig the tree up to investigate what is going on at the roots, and get a soils test for toxicity contamination if there are no obvious signs of the other problems I suggested above.

Dan's default #1: improperly planted. But without pix impossible to say with any certainty.

Dan

Or it was stuck in the pot too long before you bought it and the roots are wound around and around themselves.

These trees are fairly fast growing. The easiest thing to do is maybe just plant a new one. (Check the root system of the one you buy.)