Tenant 20 days late. Paid rent but not the fees

[Sherman Johnson]

Tenant has been renting my property for 8 months. They paid the rent on time for most the first 6 months. The last 2 months the rent has been late. The late fees were always paid per the lease agreement. However, this month the tenant was 20 days late. They paid the monthly rent amount but not pay the late fees. Until now I had no issues with this tenant. 

 Any thoughts?