can be several reasons for this:
a) You forgot to capitalise the word. In English, capitalisation matters and is important to the correct spelling of the word. Some words are always capitalised, no matter where they appear in the sentence. These words must be capitalised when you answer, or else it will not count. Eg. "Wednesday" is OK, "wednesday" is not.
b) You spelled the word correctly for American English, but your settings were for British English or vice-versa.
c) The spelling you think is correct is not included in our database - i.e. there is a discrepancy or a mistake. Please let us know if this is the case, and we will be happy to look into the matter. Don't forget to include your source for cross-referencing.