Thomas,
Here's one idea. When they refused your suggestion to write a contingent offer, you could ask them "How will you feel if the house goes under contract before we do write an offer?"
If they say they won't care, then I don't think you could generate a sense of urgency.
If they say they'll be disappointed, then you could explain that while it may seem like a small possibility to them, your experience is that it happens much more often than they might think. And, really encourage them to write the contingent offer.
You could use this most recent situation as an example. I think often buyers think that since a home has been on the market for a while, the odds of someone else making an offer to beat them out is small. But, I think that home sales go in cycles. Once the home has been promoted for a period of time, the people who are interested in buying it probably do come all at once for a period of time.
Were they pre-approved for a mortgage? Maybe that would have helped.
Kathleen