Just because you are not paying for services does not mean that services should not meet industry standards. Following a care plan, being attentive to needs and helping your mother feel safe and secure in her situation are all standards that an agency aide should meet.
Your question states that you are using Medicare-Medicaid for services. These are actually two different programs. Medicare is usually time limited and specific to an acute incident. Medicaid services, however, are usually set based on need, and a specific number of hours is recommended and provided.
To go through 20 aides is troubling, however, and tells me that there are other issues present.
I would suggest having someone from the agency trouble shoot with you as to how to better meet your mother's needs. Items that might cause problems with staffing are scheduling (perhaps you can be more flexible in request), distance (is the location accessible by public transport; if not perhaps assistance in getting the aide there would help), conditions in the home (the environment must be somewhat comfortable and compatible to the aide as well: is it too hot, stuffy? excessively cluttered? are there pets? does your mother smoke?). Determining what is the cause of the difficulty and working with your mother and the agency to solve that might assist in alleviating the problem, and get the consistency improved.