OWE rules are complex, and they are not often encountered by many agents. There are some locations where the staff are very familiar with the rules and thus you can book in this way. In other locations, the staff may see very few OWE's each year.

I'd be hestitant to call an agent incompetent simply because they don't fully understand the finer nuances of a OWE ticket.

So, one approach often mentioned is to get the ticket set up by the RTW desk of the airline, and then actually issued by the local agent. That way you have the expertise of staff dealing all the time with OWE tickets getting the thing into the system, and can then deal with the local agent for the handover of $ part.