Hi John,
I can't see anything obviously wrong with the code. However, the algorithm you are using is going to be VERY slow. For each line in the CSV you are looping through every net in every design in the buildlist. Assuming you have tens of designs and hundreds of nets, that is a lot of data to process (over and over again).
A better approach would be to read the CSV file data into some sort of datamodel in memory. You can then process the buildist designs and nets once. For each shared net, you would lookup the data in the CSV datamodel that you have previously loaded.
Simon