Western Isles MP Angus MacNeil has written to the Scottish Ambulance Service and Health Minister Alex Neil about the alleged "alarming" situation.
The SAS laid the blame on operators Bond. Both the ambulance service and the MP have written to Bond, who supply the pilots.
Nationalist Mr MacNeil highlighted the situation on the isle of Barra, where he lives, but said he wanted assurances about the situation affecting other islands that rely on the patient airlifts - especially in emergencies.
Mr MacNeil said he had heard of four recent incidents on Barra alone. It is not clear if the grounded flights involved emergencies. "It is a matter of some alarm that has come to my attention in the last few days, that on no fewer than four occasions in the last six months, the air ambulance helicopter has been unable to come into Barra at night due to pilots, it has been alleged to me, not being certified to service the island in the hours of darkness," said Mr MacNeil.
"Quite what this would be is unclear. But if this is the case, we need to understand why this certification is not in place and ensure remedial action is taken as a matter of priority so that the air ambulance cover for Barra, and perhaps other Scottish islands, is 24/7 as would be expected.
"Whether this would have affected the four previous cases will be of concern to relatives and friends but the crux of the matter is that an air ambulance service should be able, weather permitting, to access islands at all times of the day and night.
"Further to inquiring directly with the helicopter providers and the Scottish Ambulance Service, I have notified Health Secretary Alex Neil of my concerns.
"We cannot have a situation where we have a fifth such event. If an air ambulance is required, by its very nature it is emergency transportation and avoidable delays are totally unacceptable."
A spokesman for the SAS said last night that it was also worried about the situation.
"We are aware of the situation and have made it very clear to Bond that this situation needs to be rectified as a matter of the highest priority," he said.
"In the meantime, if for any reason our air ambulance cannot respond to an emergency incident, MoD or Coastguard aircraft are tasked, which is in line with normal contingencies and ensures that emergency cover is maintained."
All of the SAS's aircraft operate with paramedics on board and fly around 3500 missions annually.
The service works closely with specialist retrieval teams such as the Emergency Medical Retrieval Service, neo-natal and paediatric, and also works with Search and Rescue aircraft of the Ministry of Defence and HM Coastguard.
Unlike the rest of the UK, where air ambulance services are funded by donations, in Scotland it is funded by NHS Scotland.
A spokesman for Bond said: "To ensure the safety of all passengers, pilots and paramedics, Bond requires all pilots to complete a programme of familiarisation with this specific site before operating from it in the hours of darkness.
"Currently eight pilots based in Glasgow have been trained on this site. Two are still to receive the full process of familiarisation required. The Scottish Ambulance Service is aware of this and we are working to ensure this is resolved as soon as possible."