A mass vaccination centre in Cardiff says the vast majority of its missed appointment have been caused by GP's not having up-to-date addresses for patients.
Cardiff's Bayside vaccination centre decided to dig deeper and try and find out why people weren't attending their jab appointments after a worryingly high number of no-shows.
And they found that in most cases appointment letters had gone out to the wrong address.
Tracy Meredith, head of operations for testing and mass immunisation, said: "We've had a lot of people not attending which has been frustrating.
"After investigating this, we've noticed some people haven't updated their GP records, so the letters may be going to someone else.
"It means they may have moved or changed their details so we're urging people to get in touch with their health boards and update their information. We are phoning people to see if they still want the vaccine or to check if they've forgotten.
"We're trying to be proactive and ensure we are ringing people to check if they're coming so we can bring people's vaccine forward."
One of those people who moved house but failed to register his new details was Gareth Blanch.
The 57-year-old who had his vaccine on Thursday, said: "I moved house and didn't register and when I chased the health board, they told me I should've updated my details and I could have had it done before.
"I live down the road from the centre and I was booked in within two weeks. The whole procedure was great and I urge people to check their details and make sure they're registered with the correct health board.
"My partner gave me a kick up the backside to get myself set up and I'm glad she did. All the staff here were really helpful and all I can say is thank you because they're doing well. It's a sense of relief and I'm happy it's done."
On Monday, in one day alone at Swansea Bay, out of 1,750 appointments, 492 did not attend their appointment at the mass vaccination centres.
In a statement, the health board said: "Out of 1,750 appointments, 492 were classed as DNA (did not attend). That’s 28%. The vaccination is safe, effective and one of the ways we will overcome this pandemic.
"We urge everyone invited to attend their appointment. If you can’t make it or don’t want the vaccine, PLEASE let us know."
Clare Boland, who is a GP practice manager in Port Talbot, described how news of the no shows had left staff in tears, and she pleaded with people to call and cancel their appointments if they can't make it so others can be offered the chance of having a vaccination.
"I have had staff in tears this week over the announcement of no shows," she told BBC Radio Wales. " We have had 100% the whole way through, but there seems to be a problem with people not attending mass vaccination people are not attending.
"A portion of that is incorrect data because younger people are more transient, but in many cases people are just not turning up for the jabs and that is very concerning for the whole population moving forward.
"You are talking about people who have been giving their absolute all for 15 months now. To see them upset and frightened because they are on the frontline and looking to the day when life will get back to some sort of normality is not fair.
"They are really starting to worry about these people who are not turning up when we have worked so hard to make sure everyone is vaccinated."
There is also concern about the hundreds who have failed to turn up especially after an online booking system allowing people aged 40 to 49 to book themselves a Covid-19 vaccine crashed due to overwhelming demand.
