When a user registers and completes an online event form and the transaction is processed into Raiser's Edge, the attribute populates on their participant record. If the user comes back and registers a 2nd time, then it may add the second registration on the same participant record. When this happens the attribute from NetCommunity is not added to the record as there can be only one attribute per participant record.
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Alternative Solution Participant attributes can be recovered in the plugin by clicking on the transaction details tab before processing or by pulling raw data.