Around 44% of ambulances turning up to Lincoln, Boston and Grantham hospitals were delayed last week.
Most waited for at least half an hour, while more than 150 crews had to wait for more than hour to hand over their patients.
It comes as it emerged that England's NHS beds are running at almost 95% capacity - as experts warn of a crisis this winter.
Official stats also show more than 10,000 ambulances are waiting more than 30 minutes to get patients into hospital every week.
Another report from NHS Providers says staff shortages are making things worse.