RowSeats Core CSV upload problem

I loaded the free core at a testing site, uploaded a 2000 seat CSV with no difficulty and made test sales.
My vendor then loaded the purchased version of the core at the target site which he hosts. The sample template uploads OK but the 2000 set CSV causes the event to completely disappear from MANAGE SEATS. It still appears in the calendar.
My vendor insists there is a problem with the CSV that works on my test site.
When the file is cut in half it will upload which suggests to me the problem has to do with size..
If this is a CSV problem how do I go about isolating the problem
If not the CSV then what is the next troubleshooting step
Thank you