Please make sure that you are using the correct seed phrase, even a single space or character (even if you click on the Return/Enter button when you are importing the seed phrase) will generate another waves address. One seed phrase generates only one waves address.
Also please make sure that the original phrase doesn't have any white spaces or line breaks at the end of the line because Beta wallet doesn't recognise any additional non default symbols at the begin and at the end of the phrase.
By the way, the seed phrase always contains 15 words with spaces between each word. Except the beginning of the first word and the end of the last word.
For example (check attachments):
With one space>
With the pressed Return button
Customer support service by UserEcho