You must have changed your data source after setting up the placeholders. Check your placeholders one by one. Each placeholder you select should highlight in the Data Merge panel. The ones that dont' show need to be deleted.
I bilieve this can also be a sign of a line break in a field in the spread sheet that creates a new record where there should be none when converting to text and it gets things out of sync. You should either use multiple fileds if there are multiple ines of running text, or substitute some sort of code for the line break and then suse find/change after the merge to change the code into the break.