Simultaneous subjects

We often have multiple subjects completing our medialab experiment at the same time. We are finding that the data is being captured in the .txt file, but is not being written to the .csv file for all subjects. For instance, we had two subjects completing the same experiment at the same time and only one subject's data was written to the .csv (I assume it's the subject who began the experiment first). Is there any work around so that all subject's data will be written to the csv file as well?

Thanks
Rachel

The files are located on a common server then I'm guessing? If so, are you using "unique machine codes" to ensure that data from multiple subjects will not collide?