Camilla House in Edinburgh, run by Birmingham-based European Care, was severely criticised and served with an improvement notice after investigations by the Care Inspectorate.
Inspectors found residents who needed help to eat were ignored and other residents were only assisted by staff to drink morning tea after it had been poured for 20 minutes and was cold.
Some residents of the home had not had their teeth brushed for several days.
The Care Inspectorate has graded the quality of care provided at the home in The Grange area of the city as "unsatisfactory", the lowest possible grade, for the second inspection in a row.
The social care regulator's formal improvement notice on the service lists the immediate improvements required at the care home for up to 48 older people.
The report reveals: "Breakfast was prolonged, residents did not receive the support they needed to eat breakfast and slept at the table.
"Residents who were napping were not roused but a drink was placed in front of them, then removed untouched, the resident still napping.
"As a consequence some residents did not receive a drink even though it was a particularly warm day.
"This staff practice raised the risk that residents were not receiving enough fluids."
Inspectors said the care provided in key areas of health care were a cause for concern. There needed to be improvement in how residents' skin care, oral hygiene, hydration and pain was managed, they said.
"The recording of how residents' care is provided needs to improve so that there is clear guidance to staff about the care the residents needs and how staff must provide this. The deployment of staff at breakfast and morning tea time needs to improve."
The inspectors added: "The provider needs to get better at notifying the Care Inspectorate of significant events. The action plan, which the provider submitted following the last inspection, detailing how it planned to address the requirements made, had not progressed in the way the provider had stated."
A spokesman for the Care Inspectorate said the home would be subjected to further monitoring to ensure improvements are made.
"We continue to have serious concerns about this service," he said.
"Our report and the improvement notice we have issued to the service list the immediate improvements we require to ensure the quality of care provided to residents meets the standards we expect.
"We will be monitoring this service closely and will be inspecting again soon to check on progress.
"If we are not satisfied that urgent improvements are being made sufficiently quickly we will not hesitate to take further action.
"Everyone in Scotland has the right to safe, good quality care which meets their needs and respects their rights."
A spokeswoman for Camilla House said it was working with the watchdog to deal with the concerns that had been raised in the inspection report.
She said: "I can confirm that we are continuing to work closely with the Care Inspectorate to immediately address the areas they have raised.
"The health and wellbeing of the people we support is our number one priority, and we are working to make improvements as soon as possible."