Do I need to transfer into a different ISA each year? Can I not carry on paying into same one

[spekulatius]

Just wondering as we took an ISA out last tax year and when I tried to put more money in now it was returned into our bank account. I thought we only transfer if we find a better rate. Does anyone know?

No, you can use the same ISA, but the allowance just starts again each April 6th.
Was it definitely last financial year that you last put money in?
If so, perhaps it is a glitch in the online banking over the bank holidays, or contact the bank and ask what is going on.

Also it depends on the specific ISA terms and conditions - some are single initial payment only.
Speak to the provider.

check the rate on your old one .... most of them only have a good rate for 15 months and then drop down to 0.25% or worse

if you don't pay in for a year you usually have to reactivate them - which in the case of my bank means completing a 7 page form and posting it to them, great!

Was it definitely before 6 April last year?

Also sometimes providers need a form from you to declare that you haven't added to any other isa in the previous tax year.