I have an easy-access cash Isa with Plum. On 5 July, I requested £16,000 to be transferred out of the Isa and to my bank.

Fast forward to today and my bank, Santander, has been trying for weeks but Plum does not respond to Santander's requests for the transfer. 

They also have a weird system of NI numbers instead of normal bank accounts that causes extra confusion for no reason.

I was under the impression that money from a cash Isa should be transferred within 15 days but 1* reviews on review sites suggests others are also struggling with Plum.

Why is the transfer taking so long?

Helen Kirrane of This is Money replies: App based Plum has one of the best Isa rates on the market paying 5.17 per cent. Many savers like yourself have flocked to it for this reason. 

As you say, Isa transfers are supposed to take no longer than 15 working days for cash Isa transfers.

This is Money asked Plum if it would look into your case to see what has gone wrong and to explain more generally how it processes Isa transfers out. 

In your case, Plum told me the original transfer request was not processed when it was first received on 5 July. 

Plum assured me your transfer out to Santander has now been completed. Plum said it is looking into why this happened and will take steps to ensure this doesn’t happen again. 

This is Money also asked Plum if it had been overwhelmed by demand for its best buy Isa. 

James Blower, founder of Savings Guru said: We do see often with new providers that they don’t understand the volumes that you get at the top end of the market, nor the operational impact and how to gear up for it. 

Plum is paying a very high rate - and have been for sometime. 

The transfers out should be low in number because of the rates they are paying, I suspect they are prioritising the transfers in. 

A spokesman from Plum said: We're delighted our cash Isa has been so popular and we’ve been able to attract so many new customers to Plum who are enjoying a positive experience including a market-leading rate. 

We’d also highlight that the vast majority of customers are really happy with Plum, as evidenced by our high ratings on the App Store (4.7/5) and Play Store (4.6/5).

How long should an Isa transfer take?  

James Blower replies: All providers have 15 working days to complete transfers so they will have to report on that if they are not meeting it regularly. 

A spokesman from Plum replies: The timeframe varies, but we generally aim to process cash Isa transfers within 15 days.

Helen Kirrane of This is Money replies: The 15 day timetable does not apply to transfers from a cash Isa to a stocks and shares, innovative finance or lifetime Isa, or vice versa. As yours was from a cash Isa to your bank, I can see no reason the transfer should have taken any longer than 15 working days. 

How do plum Isa transfers take place? 

A spokesman from Plum replies: The transfer process varies between providers due to their different communication methods. However, we aim to make the process straightforward and deliver requests as soon as possible. It’s worth noting that some providers insist on sending transfer requests by post, which tends to delay the process. 

As Plum is not a bank, payments need to be routed via our e-money wallet and payment provider. That’s why we use customers’ personal details and national insurance number to identify their account, which ensures the money can be tracked at every stage of the process. Many other cash Isa providers operate in a similar way. 

We can confirm that the transfer request for this customer has been processed from our side and the money is now with Santander.

In this unusual case, the original transfer request was not processed when it was first received on 5 July. We’re looking into why this happened and will take steps to ensure it doesn’t happen again.

We'd like to apologise to this customer as we aim to process all cash Isa transfers within 15 working days.

We'd like to reassure customers that this isn't the norm for transfers out from Plum. The vast majority of transfers out of the Plum cash Isa are processed quickly and without issue.