Why doesn't a RESERVATION with United mean RESERVED seats on the plane?

Okay, what in Wright's name is going on?  (Get it?  Wright brothers. Okay, not so funny.) In the last week we've flown on 7 flights with United and it's partners and only once was I able to get seats together with my 6 and 9-year-old.  The one flight with seats together was the flight out of Belgrade to Frankfurt which is never full and there are no assigned seats.  On the 8+ hour flight from Frankfurt to Chicago in which I was flying alone with the kids, they had the three of us spread out over the three sections of the plane, and all in middle seats.  Wonderful.  So, not only did I have to negotiate for seats together, I had to do it with middle seats.  Who wants to sit in a middle seat for a transatlantic flight?  No one.  No one had pity on the lowly mother with young children spread across the plane.  One woman actually said to me, "I'm not changing my aisle seat on an 8-hour flight.  I'll look after him.  You don't need to worry."  The mama bear woke up inside me (my 3am wake-up call (did I ever really fall asleep?) wasn't helping) and it took all of my will power not to - even verbalizing my thoughts would get me into trouble.  Finally, a wonderful gentleman overheard the conversation and told me that at his wife and two children were flying to Japan at the moment (compatriots in need of karma!) and switched his aisle seat with hers and then switched that seat with my middle seat.  Wherever you are, my airplane knight, I thank you, again!

We had made these reservations 2 months prior to our travel.  Like usual, in making the reservations, they asked for the ages of the kids.  And, like usual we thought that in making RESERVATIONS we were RESERVING seats together.  Apparently, these two words no longer are related.  This has never happened to this extent before now.  The ticket counter woman kept saying that now in addition to buying the tickets you have to reserve seats.  But when we tried to "reserve" seats on flights 5 days in the future, all seats were full and they couldn't put us together.  Then we tried to reserve seats for our return to Serbia in a month.  The agent said that we couldn't; they hadn't assigned an aircraft yet.  Um.  So, when can we reserve seats?  When the aircraft is assigned.  When will the aircraft be assigned?  We won't know until it is assigned.  Am I in some kind of Orwellian airlines warp?

Spouses, does anyone know how to reserve seats together for flights with United?  I'll be damn if I have to deal with this on our way back.  I have a feeling that I'll be damned.