A woman with learning difficulties lay dead in her sheltered housing for up to six days before her body was discovered.
63-year-old Kathleen Cooke passed away on July 20, but was not found until July 26.
Her family have since raised questions regarding her death and why her body was not found sooner due to Kathleen being a resident under ADRA's Supported Housing Scheme in Morfa Cadfan, Tywyn.
North Wales Live reports that ADRA offered its "sincerest condolences" to the family and stated that it was "looking into" their concerns.
Prior to her unexpected death, Miss Cooke had been introduced to a new warden on July 19, however, when a different warden attended the premises on July 21 there was no response from her flat.
Her sister, Janet Mills, has said that the coroner had been told that the new warden was unaware that she should have entered the premises having not received a response from Miss Cooke.
Two days later a phone call to Miss Cooke's property went unanswered with staff assuming she had gone out for the day, they said.
By July 26 another resident expressed concern that she had not seen Miss Cooke for a number of days and a family friend went to the property where she sadly found that Miss Cooke had passed away.
A postmortem revealed that Miss Cooke died from left ventricular hypertrophy - also known as enlarged heart - a condition in which the muscle wall of the heart’s left pumping chamber becomes thickened.
The family have been left "traumatised" by the incident and have since raised a complaint with the housing association demanding that an investigation be carried out into the circumstances surrounding Kathleen's death.
In her letter to ADRA, Mrs Mills said: "It is evident that Kathleen collapsed on the evening of Tuesday, July 20 - her corpse was not found until 1pm on Monday, July 26.
"Whether her death was instantaneous or she was unconscious for a while can never be known.
"Kathleen was lying on her bedroom floor and after six days during the heatwave was in a rapidly advancing state of decomposition.
"This delay highlights to us a lack of diligence and awareness on the part of ADRA.
"This experience has obviously been a traumatic one for myself, the family and friends as well as other residents at Morfa Cadfan.
"I am aware that the Covid-19 pandemic has forced a number of changes to warden duties in their sheltered housing accommodation over the last year and a half, however this event shows a clear failure of the basic duties of care to their tenants - the Covid pandemic can not be used as an excuse.
"I have requested that a full investigation be carried out and recommendations and adjustments made so that other families, friends and communities will not suffer this appalling experience."
Mrs Mills says she is also "troubled" by the way ADRA handled the situation following her sister's death and says she is disappointed with the lack of correspondence from the housing association.
Mrs Mills added: "No one from ADRA has had the decency to contact me personally regarding what had happened during the six days between my sister's death and the discovery of her body.
"I am troubled by the lack of communication from ADRA after Kathleen's death.
"I have had to instigate all communications and I have still not received a reply from the Welfare department or any explanation from ADRA."
A spokesperson fro ADRA said: "We are very saddened about the death of Kathy Cooke and we would like to offer our sincerest condolences to her family and friends.
"Kathy Cooke was an active person and active within the community in Tywyn and had many friends.
"Morfa Cadfan is one of our Supported Housing schemes where people can live independently in their own flats. Our staff and residents did identify that Kathy hadn’t been seen for a few days and sadly she had passed away at her home.
"The family have been in touch to express concerns and we are currently looking into these. Our thoughts are with Miss Cooke's family and friends at this difficult time."
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