It appears that it is not possible to add more than 256 words to the Word Bank via Season Vocabulary. Here are the steps to recreate the problem:
1. Delete all words from Word Bank.
2. Go to Beginner Season 6.
3. Click on the Season Vocabulary link.
4. Verify that it says "271 Words" at the top.
5. Click the Show All link at the bottom.
6. Select all 271 words in the left column (if you select them one by one and count them up, you can verify that there are 271 words on this page).
7. Click the Add to Word Bank button.
8. Go to the Word Bank. If you select Show 50 per page, you can verify that there are five pages of 50 plus a sixth page of 6 = 256 words.
There's another kind of problem. If you repeat the above steps except using Beginner Season 1 (which has 1475 words), when you click the Add to Word Bank button (step 7), you will receive a dialog box with the following error message "An error has occured. Add words to wordbank #1". No words will get added to the Word Bank.
Thank you.