Morning 🌞

I opened a fixed rate saver a few days ago. I've had an email to say the account is up and running (already a customer so no letter needed I don't think?). I tried a small deposit of £1 just to check it was all working as I want to transfer a large sum £20,000 and didn't want it ending up lost in cyberspace. My £1 deposit has bounced back twice. Is this because it's below the minimum threshold for deposits or is there an error? It's making me nervous to deposit the large sum.
Can anyone advise please?
Many thanks 😘