Update: I didn't realize where you were timing out. Try session->waitfor('/#/') after the copy command instead of the sleep. That should avoid issues with a long TFTP copy

Might be a timing\prompt issue. Here's your copy dialog prompt:

DistrictOffice#copy tftp start

Address or name of remote host []? 172.16.0.100

Using session->waitfor('/\?/') might work.