This situation can vary from person to person, depending on if they are trying and doing what you ask (such as staying in touch) which you say she is not doing.
Since the situation has not been improving, you could try doing one of the following;
Check for any social services, low income services, elderly agencies, etc in your area (city, county or state) that may help her by paying a month's rent, helping with groceries or any other assitance that may result in money she can free up for rent.
Post a legal type notice on her door or give/mail to her, such as a 5 day notice to pay rent or whatever your state has. If rent is not paid, then an eviction may be inevitable.
Has she been with you long? Does she have a job? Is that why you accept 2 rent payments on 2 different dates? Please give a little more info, if possibly so we can understand the situation a little better. Maybe she is on a lot of expensive medications, or who knows what else. I am not saying that you can let her stay rent free, but sometimes it helps to know why they are behind.
Cat
--207.190.xxx.xxx