I sent some USDT from Kucoin and I can't use them. I can use the other USDT I have, exceeding the amount in discussion.
The contract is correct, Phantom lists them on another line and Solflare mixes them with the rest of the USDT.
The only potential issue I noticed with the token account in question is that is appears OFFcurve.
Any suggestions?
no actions taken so far, I understand that they can be moved on curve
I don't think the token account being off the curve is the issue here. If by token account you mean the Associated Token Account (ATA) of your token then it is supposed to be off the curve as it is a PDA. It would be great if you can share the explorer link for the USDT contract you are using and address of the token mint account. I don't have enough reputation to write this as comment so had to put it as an answer.
Also check if the ATA account has been freezed by you in which case you cannot perform any transaction until you thaw(unfreeze) the account.
Try putting this question in Solana Stack Exchange where someone might give you a better solution.