When a tenancy is signed later than the start date, you may need to make some manual adjustments because the standard process is that when a tenancy is signed, it will first call for any bond that was preset against that tenancy. It will then follow the bond by calling for the first month's rent, and finally any auto-charges you may have applied to trigger upon signing.
However, if the tenancy is signed more than 21 days after the start date, it will not call for the rent due in that period.
For example, if you have a tenancy due to start on 1st of March and the tenancy is not signed until the 25th of March, you will need to ensure that any rents which did not automatically call are made due manually so that the landlord does not miss any rent payments.
To make rent due manually, please follow this guide - Make Rent Due