Asking existing tenant to resubmit their proof of income

[Jackson I.]

Question for the landlords....

Asking existing tenant to resubmit their CURRENT proof of income...

My rental is in Maryland. I just started having problem with one of my tenants that has been good for two years. She and her father have been struggling to pay the rent for the last three months. However, as of now, they are only behind for this month. A lot of phone calls and texts have to happen in addition to me serving her "Notice to Pay or Quit" before she quickly goes and find a fraction of the amount due and pays. She will then promise to pay the rest as soon as she has it. Is there a legal way to ask her to resubmit a current proof of income? In addition, should I be asking for current proof of income each time the lease is due for renewal? 

Thanks for your input.