Also agree with the above.
If you are handling and preparing the paper with bare hands, any contact may leave traces of oils from your skin on the paper. These oily spots preventing the sensitizer from properly penetrating the paper fibres in that area, leading to loss of image metal during processing.
Or the paper just needs a little help to ensure the sensitizer gets absorbed, so adding a drop of dilute surfactant helps the coating process.
So. If you do not wear gloves, get some and wear them during all handling of the paper prior to and during sensitisation. Then if that does not work (or you already wear gloves), try adding some non-ionic surfactant to the prepared sensitiser you are about to apply.
If neither of those work, then it is time for another paper.