I expected to receive a .digicash payment file but I haven't received an e-mail yet.

Contact the payer and make sure that they sent the .digicash payment file to the correct e-mail address. 
Make sure the e-mail address they used is the same as your digi.cash account e-mail address. 

If this is not the issue, please put in a formal claim via the Support Centre and we will contact you as soon as possible to resolve the issue.