collecting late fees from tenant

[Jason Schatz]

I have been reading a lot of the good posts on here about structuring of late fees and it has been helpful.  My leases currently have rent due on the 1st with a 5 day grace period and a flat late fee of $50 after that.  My rent collection is done online and the service automatically issues the $50 late fee if rent is not paid by the 5th. 

My question is, I have a tenant who has been late twice and the late fee has been added to their account but they are only paying the monthly rent.  How do you all force the collection of your late fees?  I could reach out with a reminded that they have an un-paid late fee balance but if that is ignored what is next?