Hi, first let me start with the obligatory 'if this has been posted before, I'm apologize' apology.

Seriously, I searched but got no where so...
... with that out of the way, can someone clue me in:
I have been attempting to purchase an economy plus seat on one of my flights - this particular leg being from Melbourne to Houston on Oct 21, 2014. Segments of this leg are:
MEL-SYD UA840(for flight change)
SYD-LAX UA840,
LAX-IAH UA1180.
Since money is funny, no 1st class flying over here which is why economy plus in seat 21C would be fine.
In fact, right after I booked my flight, I noticed the option to purchase the economy plus seat from SYD-LAX for $10, but could not select anything in this section. So I moved to the MEL-SYD segment, and was able to purchase seat 21C for $10. I see now it was a waste of money because...
...when I went BACK to the SYD-LAX flight, I still could not purchase econplus. I was able to select seat 34A in economy. But now, when I go back to this leg of my reservation, I can't even switch this seat out to another economy seat on the plane now. I hit 'continue' and things just don't update.
So I called and the agent says that the flight is too far away to make a selection. I'm like, ok, whatever. It maybe true but it wasn't the answer I wanted to hear so I gave her a piece of mind. Actually, I just politely thanked her and hung up. lol
But I go to check my last segment, LAX to IAH, and I am able to purchase E+! I even tried to see if the same thing would happen if I purchased MEl-IAH as a one way by itself - not that I would - and the same thing happens: SYD to LAX is blocked out.
What is interesting though - and this is where I think it gets glitchy - the price of the econ+ seats for the SYD-LAX segment seem to depend on which other segment I click on first. Here is what I mean:
When I load my ACTUAL reservation, if I click on the first leg, all E+ seats are 'from $97.00'. Even when I move over to the SYD-LAX leg the E+ seats are also 'from $97.00'.
But, if I click on the last leg, the E+ seats change to 'from $47.00'! Then I go back to SYD-LAX, and although I cannot make a purchase, the econ+ seats now show as 'from $47.00'.
So I cannot understand:
1) why I cannot change the seats (or purchase an economy + seat) on my SYD-LAX leg and
2) if I am finally able to purchase the seats, will the price start at $9.00 or $47.00 or $94.00? Its no big deal if I can't but since I am not a frequent traveler, I thought I'd check here to see if this is expected behavior.
So, has this happened to anyone else? Any tips for this fresh newb?