Tenants paid full rent late, but have not paid late fee

[Dar Fornelli]

For the second month in row, my tenants have paid rent 3+ days late. My lease states the following: 

Shall tenant(s) accounts not be paid in full by 4:00 pm on the 2nd day of the month, a late charge of twenty five dollars ($25.00) will be added to the outstanding balance to cover costs of collection, and if payment is not received thereafter an additional late charge of ten dollars ($10.00) per day will be added to the unpaid balance.

They have since paid the monthly rent value, but not the late fee totaling $45. Now they are non-responsive to calls and texts. Should I continue to tack on the $10 late fees since there is still an outstanding  balance? How long do I give them before taking legal action? 

Appreciate any input!