1) Make sure you're using the correct email address: many people have multiple email addresses, and try to reset their password using the wrong address (e.g. their Gmail account instead of their .edu account).
2) For Gmail users: make sure you're typing your email address exactly as it was entered when you signed up. Even though Gmail supports optional periods in your email address (i.e. "firstname.lastname@example.org" and "email@example.com" go to the same email inbox), Splitwise currently treats each unique spelling of your address as a separate account.
3) Be patient! Password reset emails may take a few minutes to appear in your inbox. Please note that every time you request a new password reset email, it invalidates all previous password reset emails that you have been sent as a security precaution. This means that only the most recent password you have been sent will work.