We have had our housekeeper for years (we do not tip her---she is paid very well), but when she has been out of town, I have occasionally used another service.
I have never tipped the other housekeepers because the fee we paid them was agreed upon prior to beginning the service and was consistent with the going rate for a house of our size.
I would not tip a housekeeper any more than I would tip a plumber, a painter, or any service person that came to perform a specific service at our home.
If she expects tips and is continally disappointed, then you might suggest she consider raising the rates for her services.
ETA: We do give our housekeeper a monetary gift at the holidays, but it is a gift, not a tip or a bonus. She owns her business and also does the work herself. We also give our landscaper (who also owns the business and does the work) a gift at the end of the year as he always seems to go above and beyond taking care of everything---especially as he works in extreme heat conditions in our desert summers.