The super low unemployment rate for menial labor is part of the problem.  Lot's of ultra-low-paying jobs are available, so most of the menial labor market has two or three low-paying jobs + Uber/Postmates driving just to survive. There is always someone willing to pay $0.50 more per hour or who is closer to home - so less commute.  This might be the case for many of your cleaners.  

How to retain them? Well, make time-and-a-half overtime opportunities the norm so that they work harder but avg out closer to double their current income.  Offer gift cards to Target and Walmart if the company gets a good review that mentions a cleaner by name.  Offer them all an oil change once a year or fill their tanks once a month (if they have cars). Do you have >10 employees? See if you can onboard a PEO solution that saves you money on workers comp or EPLI to make some of these perks possible. Hope that helps.