Can't get PSOWeighting to work

I've been playing around with PSOWeighting, and must be doing something wrong, because however I run it, it is returning weights of zero for every attribute.

Here's the fragment of the operator chain:

        <operator name="PSOWeighting" class="PSOWeighting" breakpoints="after" expanded="yes">
            <operator name="XValidation (2)" class="XValidation" expanded="yes">
                <parameter key="number_of_validations"	value="5"/>
                <parameter key="sampling_type"	value="shuffled sampling"/>
                <operator name="NearestNeighbors" class="NearestNeighbors">
                    <parameter key="k"	value="2"/>
                    <parameter key="keep_example_set"	value="true"/>
                    <parameter key="weighted_vote"	value="true"/>
                </operator>
                <operator name="OperatorChain (2)" class="OperatorChain" expanded="yes">
                    <operator name="ModelApplier (2)" class="ModelApplier">
                        <list key="application_parameters">
                        </list>
                    </operator>
                    <operator name="Performance (2)" class="Performance">
                        <parameter key="keep_example_set"	value="true"/>
                    </operator>
                </operator>
            </operator>
        </operator>

My data has both numerical and polynominal attributes, and a numerical label.

I was originally running the chain with EvolutionaryWeighting (after a suggestion I found on this board), and it successfully returns non-identical weights.  Replacing the operator with PSOWeighting, and using the default parameters, produces all zeros.

I saw no errors in the logs, and looking at the docs there don't appear to be any missing inputs.  What am I missing? 

Thanks, as always.

Keith