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What could prevent the seat assignment?
Note that United has E+ which is can only be assigned to UA/CO eltie members + people who paid for the "upgrade".
Do both of you have status (you said husband is Gold)? Is the status flyers' FF# in the system? Are you trying to get exit seats (with a lap child)? Have there been equipment changes?
The poster above said that linking the PNRs is only a notation for the agents, the computer does not undrstand that.