Which step did you do so far, my guess is you stopped after step 4? Note that you have to complete the transfer process within 2 hours or you have to start over again. So in summary I think you accepted the transfer (signed it) but did not create a response file where you copied the reponse text, changed the file extention to .slatepack, and did not upload this to Bittrex. Without the sending this response to Bittrex, they cannot finalize the transaction, meaning it will never happen.
So I think you have to start the process again from step 1)
See below a copy of the steps:
1. Go to Bittrex, Click Withdrawals under Holdings, Enter a withdrawal amount and click “Generate Transaction File”. On the Withdrawal Transaction File screen, click Download Transaction File and save it to your computer.
2. Go to the folder where you saved the file. Right click on the file open with > Notepad > in notepad, select the whole text (press simulations Ctrl + A) then copy the text (Ctrl+ C),
3. In Grin++ go to the Receive tab, then click in the left textbox with Receive above it, paste the text (Ctrl + V) and click the receive button below the text box.
4. Now in the right textbox you see the Response slatepack text. Copy this text by selecting all (Ctrl + A) and copying it (Ctr + C).
5. Open a new text file, called it response.txt or something like that, past the text and safe the file. Change the file extention from .txt into .slatepack
6. Go to Bittrex select the response.txt file you just created, and click finalize in Bittrex.
You have to do the above steps within 2 hours, otherwise the transaction gets cancelled automatically, meaning you have to start the procedure again from step 1).
Let me know if the above steps work for you.