The printer may be timing out before receiving all of the labels' data. Are all the labels unique? If there are repeat labels in the mix, you could try adding the ^PQ command to have one label printed multiple times. This would decrease the amount of data being sent across the connection. More information on the ^PQ command can be found on page 296 of the ZPL Programming Guide.
Hi Reed, What is the total amount of data you are sending? Are random labels not being printed or is it just printing the first 70+? The printers do have a buffer for print data that you might be overloading. It also may be a network issue.