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Were they having technical trouble or just having trouble finding a cost-efficient solution?

AFAIK the technical solution (Urea injection) works fine, but it's expensive and VW didn't want to include it in all cars.

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VW uses urea injections, but they wanted to make it transparent to the end user. The urea was to be refilled during maintenance, and disconnecting the system was the only way to make the tank last that long.

Not all TDI that were part of the emissions mess use urea, they also have a system that injects diesel into the particulate trap and burns out all the particulates at high temperature. Look up DPF regen. Note: I owned one of these cars, this is firsthand knowledge.

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