Recently have had a few customers saying they've not received their direct debit mandate request emails, resent but still nothing, had to set them up from scratch again?

Hello helen6, 

Welcome to the Community page, 

The only thing can think that could be preventing it from them getting the email is that the email address is incorrect, it has gone into their spam or junk box or blocked by the email provider. 

Can you and your customers check these. 

You could try this as well, set up the mandates through GoCardless direct then import the set-up mandate from GoCardless onto Quickbooks.