My Oroblanco seems kind of a little sickly as well.

It might be that since Oroblanco has more pomelo parentage in its genetic pool than your average grapefruit, that might be making it a little less hardy in less than optimal climates when the winter temperatures start dropping.

You see, Oranges, Grapefruit, and Oroblanco are all on a continuum between mandarins and pomelo. Genetically speaking, Orange is about 25% pomelo and 75% mandarin (it is believed). Grapefruit originally resulted from a hybrid between pomelo and Orange (and so very much resembles pomelo in appearance and flavor, though there are distinct differences). And finally, Oroblanco is a hybrid between Pomelo and Grapefruit. So you can see that it is nearly a pomelo (although technically it is considered a grapefruit).

Anyway, the more pomelo parentage a hybrid has, generally the less cold hardy it will be. Before the advent of grapefruit, there were very few parts of the U.S. where pomelos could have been successfully commercially grown. That is probably the reason why grapefruit has become so well known in the U.S. and pomelo hasn't.

I'm in no way suggesting you can't grow pomelos or Oroblanco where you live, I'm just suggesting a possible reason they may not be doing as well as the other citrus. In fact, summers are so long and hot in Indio that it is one of the few parts of the country where pomelos will be able to ripen to maximum sweetness.