Hey, we had this issue for ages and lost out on so many sales.
With no help from Shopify or the bank we think this may have been the issue. That we didn’t have our name on customers bank statements when the transaction goes through.
Go to settings > payments > manage > scroll to ‘customer billing statement’ and make sure your statement descriptor is correct. Ours was a bunch of numbers and then we changed it to our URL and this seemed to (so far) get rid of soooo many declined payments and loss of customers. It has only been a couple of days but we haven’t had any declined compared to every day at least 15% of sales.
Let us know if this helps anyone