I think I have the same problem as @JonnyT ?!?
I had my ADA in an old Daedalus version (WIN7) and now had to set up the PC with WIN10 in order to be able to use the new Deadalus version.
During the installation (blockchain synchronization was not yet completed) I added my old wallet (paper wallet; 27 words).
The synchronization was completed overnight, but my wallet is empty.
“+ wallet” → after I entered my 27 words (again), it says “Wallet you are trying to restore alrady exists”.
I entered my old address at cardanoscan.io and explorer.cardano.org
Both show 0 ADA. 
The last transaction was 4. Feb. 2020
I know how many ADA I should have.
When I track the transaction on the blockchain, I end up at a wallet address with exactly this amount of ADA and 1 (ingoing) transaction.
But that’s not my (old) address and I have no idea how to get my ADA now.
Any suggestions what to do?
P.S.:
Yes, I’m absolutely certain that my 27 words are correct and that I typed them correctly. I had to reinstall my PC in 2018 and restored my wallet with these words.
P.P.S.:
I doubt anyone knows my 27 words. (Living alone and PC is “clean”.)