The concatenated key you have (rptname_participantid_submitted) obviously presents a challenge. Is there any reason they can't be split up into three distinct fields?

Otherwise, is the format universal where the participantid is always separated by underscores? If so, you can make a calculated field to suss that out using a MID function.

Do participants only have one report each? If so, you can put the above MID function in column A of sheet B and then use a VLOOKUP to find the report name.