Inspectors said outstanding concerns remained from earlier inspections at Lorne-bank Care Centre in Hamilton, despite some evidence of improvement.
The service has been graded "weak" for the quality of care and support it provides at two inspections in a row.
A spokesman for the Care Inspectorate said: "We remain concerned about the quality of care and support experienced by residents at this service.
"We have informed the service of the areas which must improve quickly or we will not hesitate to take further action.
"The service provider must ensure that personal care plans accurately reflect the needs of service users and also detail how these needs will be met."
After the most recent inspection in March, it was noted that while some improvements had been made in addressing the residents' care and support needs, four requirements for improvement remained outstanding.
The spokesman added: "Adequate, trained and competent staff must be on duty at all times to ensure people receive their medication at the correct time and the provider must ensure that service users have a supply of their prescribed medication at all times.
"Staffing levels must also be reviewed in line with the dependency levels of people who live in the service.
"We will be insp-ecting this service again soon to check on progress."