Aside from nature, there are usually good reasons why headstones go unkempt, some family members may lead a busy life, are not physically capable, live too far from the cemetery or may not know the cemetery's responsibilities.
For those not familiar with a cemetery's responsibility versus the family's responsibility, I'll explain. Your headstone/memorial area is your private property when you are in a City owned and some private owned cemeteries. So, you are responsible for your family memorial and the area within any coping or sealed off area from the general cemetery grounds. Cemeteries are usually responsible for their grounds. Every cemetery has a plan with bylaws that branch off of the County and State laws. States are under the National Plan. When your cemetery's general area grounds are not maintained you can report the cemetery to your State Cemetery Recorder office.
It was challenging in the beginning! There is abundance of good and bad information available, especially on the internet. I had no idea where to begin. But thought: "How hard can it be to start a company that cleans headstones?" Well, here is where research was applied. Some information works, some doesn't. Some folks know what they are doing, others I'm not sure. After much trial and error. I ended up with what worked for us and our clients.
First Rule. Do no harm. Evaluate the condition of the stone. A simple cleaning with water and a brush may be all you have to do to your family memorial. However, weather plays a big part in maintaining memorials and the stone may require further cleaning (see Why Clean Memorials). It may also be in such bad condition that to put a brush to it would destroy it.